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CO-ORDINATION COMPOUNDS AND ORGANOMETALLICS

Contents

1. Co-ordination Compounds and Organometallics

1.1 Addition Compounds

1.2 Double Salts and Coordination Compounds

1.3 Complex ion

1.4 Ligands

1.5 Chelates

1.6 Coordination Number

1.7 Naming of the Complex Compounds

1.8 Isomerism in Coordination Compounds

1.8.1 Structural Isomerism

1.8.2 Stereoisomerism

1.9 Effective Atomic Number

1.10 Werner Theory of Coordination Compounds

1.11 Bonding in Complex

1.12 Crystal Field Theory

1.13 Colour and the Spectrochemical Series

1.14 Stability Constants and Stability of Coordination Compounds

1.15 Kinetic Consideration of Ligand Displacement

1.16 Applications of Coordination Chemistry

1.17 Organometallic Compounds

1.18 Bonding in organometallic compounds

1.19 Importance and Applications of Coordination Compounds

1.20 Importance and Applications of Organometallics

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1. Co-ordination Compounds and Organometallics

The branch of chemistry which deals with the study of co-ordination compounds is called co-ordination chemistry.

1.1 Addition Compounds. The compounds formed by the combination of two or more stable species in stoichiometric amounts are called addition compounds or molecular compounds. These can be further divided into two categories; namely, double salts and co-ordination compounds.

1.2 Double Salts and Coordination Compounds • When ������� solution is treated with KCN solution, species formed, no longer, gives tests of �� and � �� • When ����� solution is treated with ��������� solution, species formed gives tests of + 3+ 2-

4K ,Al and SO .

2 2Fe(CN) 4KCN Fe(CN) .4KCN+ →

(A)

2 4 2 4 3 2 2 4 2 4 3 2K SO +Al (SO ) +24H O K SO .Al (SO ) .24H O→

(B)

We call them addition compounds of

Type I: those which lose their identity in solution (double salt) as alum (B) Type II: those which retain their identity in solution (complexes) as potassium ferrocyanide(A)

Most d-transition metal ions have vacant d-orbitals that can accommodate electron pairs. Many act as Lewis acids by forming coordinate covalent bonds in coordination compounds (coordination complex ions).

A complex ion is a metal ion with Lewis bases (ligand) attached to it through coordinate covalent bonds. (also called coordination). A complex (or coordination compound) is a compound consisting either of complex ions with other ions of opposite charge.

4 6 K |Fe(CN) |

Complex anion +K is other ion

3 4|Pt(en) |Cl

complex cation -Cl is other ion

2 2|Pt(en) Cl |

neutral complex

Many important biological substances are coordination compounds. Haemoglobin and chlorophyll are two such examples. Haemoglobin is a protein that carries 2O in blood. It

contains iron (II) ions bound to large porphyrin rings. Chlorophyll is necessary for photosynthesis in plants. It contains magnesium ions bound to porphyrin rings. Vitamin 12B

is a large complex of cobalt.

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1.3 Complex ion. It is an electrically charged species in which central metal atom/ion is surrounded by suitable number of negative ions or neutral molecules which are called ligands. The compounds containing complex cation are cationic complexes.

The compounds containing complex anion are anionic complexes.

The compounds which do not ionize in aqueous solutions are neutral or molecular complexes.

3 3 32 3 2 [ ( ) ][ ( ) ] [ ( ) ] CoCl NHK Ag CN Ag NH ClNeutral complexAnioniccomplex Cationiccomplex

+ − + −

Compounds containing very stable complex part i.e., the part which does not dissociate in solution are also called perfect complexes or penetration complexes.

Compounds containing the complex part which has tendency to ionize reversibly in aqueous solution are called imperfect or normal complexes.

4 44 6 6 6[ ( ) ] 4 [ ( ) ] ; [ ( ) ]

( )

K Fe CN K Fe CN Fe CN

Perfect complex

+ − −→ + least tendency to dissociate in

solution.

3 3 33 6 3 3 6 3 6 3[ ( ) ] 3 [ ( ) ] ; [ ( ) ] 6

(Im )

Co NH Cl Cl Co NH Co NH Co NH

perfect complex

− + + +→ + +� ��� ��

1.4 Ligands A ligand is a species that is capable of donating an electron pair(s) to a central metal or metal ion. It is a Lewis base. In accepting electron pairs, the central ion acts as a Lewis acid.

• Ligand is said to be unidentate if it has only one pair of electrons that it can donate. • Ligand is said to be bidentate, tridentate, tetradentate etc. depending on the number of electron-pairs that it can donate. Table 2.3: Some Common Unidentate Ligands

Neutral Anions (replacing ide by O) Anions (replacing e by O)

Formula Name as Ligand

Formula Name as Ligand

Formula Name as Ligand

2OH aqua -F (fluoride) fluoro 2-4SO sulphato

3NH ammine -Cl chloro 2-2 3S O thiosulphato

CO carbonyl -Br bromo 2-3CO carbonato

NO nitrosyl -I iodo 2NO nitro*

32 NH CH methylamine 2-O (oxide) oxo -O NO nitrito*

5 5NC H or Py pyridine -OH (hydroxide) hydroxo -S CN thiocyanato*

2 5 3N(C H ) triethylamine -CN (cyanide) cyano* -N CS isothiocyanato*

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3 3N(CH ) trimethylamine

NC isocyano

6 5 3P(C H ) triphenyl phosphine

2S − sulphido

3PH phosphine H− hydrido

(a) 3:NH , one electron-pair available for donation unidentate.

(b) two electron-pairs available for donation bidentate.

Table 2.4: Some Common Multidentate Ligands (Chelating Agents) Abbreviation Multi Name Formula Charge en bidentate ethylenediamine

0

ox bidentate oxalate ion

-2

gly bidentate glycinate ion

-1

DMG bidentate dimethylglyoximate ion

-1

dien tridentate diethylene-triamine 0

EDTA hexadentate ethylenediamine tetraacetate ion

-4

Example: (ox)2- is which type of ligand?

Solution: Two co-ordinate sites are present in the oxalate ion and therefore it is known as bidentate ligand.

• Some ligands can be coordinated to the metal or metal ion through either of the sides. They are called “ambident” ligands.

If the nitrite ion is attached through N atom 2(-NO ) , it is nitro; if attached through O

atom (-ONO) , then nitrito,CN(cyano) & NC(isocyano),SCN(thiocyanide) &

NCS(isothiocyanide)etc. • Ligands can be

- Neutral molecules such as 2H O

- Anions such as -CN , 2-2 4C O

- Cation [but in rare cases since an electron pair on a cation is held firmly by the positive charge and would not be involved in coordinate bonding].

• Ligands forming complex with metal do not ionize. • In some cases it is not possible to identify donor atoms, because the bonding electrons

are not localized on specific atoms. Ethylene 2 2(CH =CH ) bonds to transition metal

through the electrons in their double bonds.

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1.5 Chelates

Some ligands are capable of donating more than a single electron pair, from different atoms in the ligand and to different sites in the geometric structure of a complex. These are called multidentate ligands. When the bonding of a multidentate ligand to a metal ion produces a ring (usually five-or six-membered), we refer to the complex as a chelate (pronounced-keylate). The multidentate ligand is called a chelating agent and the process of chelate formation is called chelation. The number of such ligating groups indicate the denticity (unidentate, bidentate…) of the ligand.

nickel (II) dimethyl glyoximate (chelate)

[bis-(dimethylglyoximato)nickel (II)]

Copper (II) glycinate (chelate)

[trans-bisglycinato copper (II)]

1.6 Coordination Number

The coordination number (C.N.) of a metal atom in a complex is the total number of bonds the metal atom forms with ligands. Some common coordination numbers of metal ions are summarized in Table.

Table 2.5: Coordination Number (C.N) of Metal Ions Metal C.N

+Ag 2

+Cu 2,4 2+Cu 4,6 +Au 2,6

2+Ca 6 2+ 3+Fe , Fe 6

2+Co 4,6 3+Co 6 2+Ni 4,6 2+Zn 4 2+Al 4,6 2+Sc 6 3+Cr 6 2+Pt 4 4+Pt 6

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Example of complex of various C.N. are given in Table below.

Table 2.6: Example of Complexes of Various Coordination Numbers

1.7 Naming of the Complex Compounds

• Naming of complex compounds is based on set of rules recommended by IUPAC.

(Name of cation) (Name of anion)

• For naming the complex part the following sequence is followed:-Name of ligands-Name of the central metal (O.N. of metal)

• For more than one similar ligands, the numerical prefixes di, tri, tetra, etc. is added before its name. However, if the words di, tri, etc., already appear in the name of the ligand then numerical prefixes used are bis (for two) tris (for three) and tetrakis (for four).

• For dissimilar ligands, the names are written in alphabetic order of preference.

• If the complex part is anion then the name of the metal ends with suffix ate.

• While naming ligands, the names of negative ligands end with ‘O’ ; names of positives ligands end with ium and names of neutral ligands are as such.

• The name of the complex part is written as one world.

• While naming the bridged complex, the names of the bridging ligands are separated by -µ- by placing hyphens on both sides.

Some examples are:

Complex compound Name

3 6[ ( ) ]K Fe CN Potassium hexacyanoferrate (III)

3 2 2[ ( ) ]Pt NH Cl Diamminedichloroplatinum (II)

2 4 2[ ( ) ]Cr H O Cl Cl Tetra aquadichlorochromium (III) chloride

2 4[ ( ) ]K Ni CN Potassium tetracyanonickelate (II)

3 6[ ]Na AlF Sodium hexafluoroaluminate

Complex C.N. +

3 2[Ag(NH ) ] 2

3-[HgI ]

3

2-4 4PtCl ,Ni(CO) 4

3-5 5Fe(CO) ,[Co(CN) ] 5

3+3 6 6[Co(NH ) ] ,W(CO) 6

3-7[Mo(CN) ] 7

4-8[W(CN) ] 8

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3 2[ ( ) ]Ag NH + Diamminesilver(I) ion

3 4 3 2[ ( ) ]( )Zn NH NO Tetraaminezinc (II) nitrate

4[ ( ) ]Ni CO Tetracabonylnickel (0)

12[ ( )]Au CN − Dicyanoaurate (I) ion

2 6[ ]Na SiF Sodium hexafluorosilicate (IV)

3 12( )Mn CO Dodecacarbonyltrimanganese (0)

5 5 2( )Fe C H Biscyclopentadienyliron (II)

4LiAlH Lithium tetrahydroaluminate (III)

[Pt(py)4] [PtCl4] Tetrapyridinplatinum (II) tetrachloroplatinate (II)

Example: Write the IUPAC names of the following coordination compounds:

(i) [Co(NH3)6]Cl3,

(ii) [Co(NH3)5Cl]Cl2,

(iii) K3[Fe(CN)6],

(iv) K3[Fe(C2O4)3],

(v) K2[pdCl4],

(vi) [Pt(NH3)2Cl(NH2CH3)]Cl.

Solution: (i) hexa-amine cobalt(III) chloride.

(ii) Penta amine chlorido cobalt(III) chloride.

(iii) Potassium hexacyano ferrate(III).

(iv) Potassium trioxalate ferrate(III).

(i) Potassium tetrachlorido palladate(II).

(ii) Diammine chloride(methylamine)platinum(II) chloride.

Example: Write the formula for the following coordination compounds: (i) Tetra-amine diaqua cobalt(III)chloride. (ii) Potassium tetracyanonickalate(II). (iii) Tris (ethane -1, 2- diamine) chromium(III) chloride. (iv) Amine bromidochloride a nitrito-N-platinum(II). (v) Dichlorido bis(ethane-1,2-diamine)platinum(IV) nitrate. (vi) Iron(III) hexacyanoferrate(II).

Solution: (i)Tetra amine cobalt(III) chloride- [Co(NH3)4(H2O)2]Cl3.

(ii) Potassium tetracyanonickalate(II).

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K2[Ni(CN)4] (i) Tris (ethane -1, 2- diamine) chromium(III) chloride.

[Cr(en)3]Cl3

(ii) Amine bromidochloride a nitrito-N-platinum(II). [Pt(NH3)BrCl(NO2)] (iii) Dichlorido bis(ethane-1,2-diamine)platinum(iv) nitrate. [PtCl2(en)2](NO3)2.

(iv) Iron(III) hexacyanoferrate(II). Fe4[Fe(CN)6]3

1.8 Isomerism in Co-Ordination Compounds

Co-ordination compounds exhibit following types of isomerism: Structural isomerism and Stereo isomerism.

1.8.1 Structural Isomerism. Different types of structural isomerism shown by co-ordination compounds are as follows:

Ionisation Isomerism. It is exhibited by the co-ordination compounds with same molecular formula but different ions in the ionisation part. For example,

3 5 4 3 5 4[ ( ) ] [ ( ) ]Co NH SO Br and Co NH Br SO

(Red) (Violet)

Linkage Iomerism. It is exhibited by the co-ordination compounds containing ambidentate ligands.

3 5 2 3 5[ ( ) ] [ ( ) ]min ( ) min ( )

( ) (Re )

Co NH NO Cl and Co NH ONOCl

Penataam enitrocobalt III chloride Pentaam enitritocobalt III chloride

Yellowbrown d

Co-ordination isomerism. It is shown by the complexes in which cation and anion part is both complexes. It involves exchange of ligands between complex cation and anion.

3 6 6[ ( ) ][ ( ) ]III

Cr NH Co CN And

3 6 6[ ( ) ][ ( ) ]III

Co NH Cr CN

Hydrate isomerism. In this type of isomerism, the isomers have different number of water molecules in the co-ordination sphere and outside it.

2 6 3 2 4 2 2 2 3 3 2[ ( ) ] [ ( ) ] .2 [ ( ) ].3CO H O Cl Co H O Cl Cl H O Co H O Cl H O

Co-ordinate Position Isomerism. This type of isomerism is shown by bridged complexes and arises due to different position of attachment of ligands

Polymerisation isomerism. It is not a true isomerism. It is used to denote compounds having same empirical formula but different molecular formula. For example,

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3 2 2 3 4 4[ ( ) ] [ ( ) ][ ]Pt NH Cl and Pt NH PtCl

1.8.2 Stereoisomerism

Stereoisomerism can be divided into two categories:

(a) Geometrical isomerism (b) Optical isomerism

(a) Geometrical isomerism

In co-ordination compounds this isomerism arises due to different geometrical arrangements of ligands around the metal atom. The adjacent positions are called cis while opposite positions are called trans.

This isomerism is common in complexes with co-ordination number 4 and 6.

(i) Geometrical isomerism in complexes with C.N. = 4. Tetrahedral complexes do not show geometrical isomerism.

Square planar complex with general formulae: 2 2MA X ; 2MA XY ; 2MABX ;MABXY ;

2( )M AB can exhibit this type of isomerism. A, B are neutral monodentate ligands; X, Y are

negative monodenate ligands and (AB) is unsymmetrical bidentate ligand.

MABXY:[Pt(NH3)(py)NO2Cl].Three geometrical arrangemants are possible for this type of complex.

M(AB)2: [Pt(gly)2]

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(ii) Geometrical isomerism in complexes with C.N.=6

Such complexes have octahedral geometry. The complexes with following general formulae show geometrical isomerism :

4 2 4 2 2 2[ ];[ ] ;[ ( ) ];[ ( ) ]MA B MA XY M AA X M AA XY

Here, (AA)2 is symmetrical bidentate ligands.

The compounds of the type 3 3[ ]MA X also show geometrical isomerism which are called fac

(from facial) isomer and mer (from meridional) isomer. In addition to these geometrical isomerism is also shown by 2 2 2[ ]MA X Y .

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Octahedral complex of the type [MABCDEF] have 15 geometrical isomers including a pair of enantiomers. However, only three isomers have actually been isolated for

3 2[ ( ) ( )( ) ]Pt NH py NO Cl Br l .

(b) Optional Isomerism

Optical isomerism is shown by the complexes having chiral structures. They rotate the plane of polarized light in opposite directions.

(i) Complexes with C.N. = 4.

Square planar complexes do not show optical isomerism.

Tetrahedral complexes with formula MABCD or M(AB)2 show optical isomers.

(ii) Complexes with C.N. = 6

Octahedral complexes with general formulae 2 2[ ( ) ]Cis M AA X Y− ; 2[ ( ) ]Cis M AA XY− ;

3[ ( ) ]M AA exhibit optical isomerism.

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Example-If A,B,C, and D are four different ligands,

(a) How many geometric isomers will be found for square planar 2+[PtABCD]

(b) Will tetrahedral 2+[ZnABCD] display optical isomerism?

Solution- (a) Following three different isomers is possible for the square planar 2+[PtABCD]

(You may write others but they will be identical)

(b) There is chirality and enantiomers do not superimpose hence, 2+[ZnABCD]displays optical isomerism.

Example-Consider the following isomers of 3 2 4[Cr(NH ) Cl ]

(a) Label the isomers as cis or trans (b) Which isomers are identical and which are different? (c) Do any of these isomers exist as enantiomers?

Solution- (a) I, IV cis; II, III trans

(b) I and IV are the same; II and III are the same

(c) None of the isomers exist as enantiomers because their mim images are identical.

1.9 Effective Atomic Number

Each ligand donates and electron pair to the metal ion, thus forming a coordinate bond. Transition metals form coordination compounds very readily because they have vacant d-orbitals which can accommodate these electron pairs. Metal ion in the complex tends to attain nearest stable inert configuration by gaining electrons from ligands. Effective atomic number (EAN) of the metal in a complex is given by

EAN=Z-(O.N.)+2(C.N)

(Where O.N. is oxidation number C.N. is the coordination number and Z the atomic number)

EAN can also be given by

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EAN=Z-(O.N.)+2(Ligands)

Assuming that each ligand is unidentate.

EAN of some metals in complexes is summarized in Table.

Table 2.7: Effective Atomic Numbers of Some Metal in Complex

1.10 Werner Theory of Coordinattion Compounds

• Beginning of the coordination chemistry is not based on the first formation of

3 2 4[Cr(NH ) Cl ]by Tassqert but is actually considered based on the outstanding work of

Alfred Werner in 1892.

• Werner prepared and isolated many different coordination compounds from like

3 3CoCl and NH

(a) 3 3 3 6 3CoCl .6NH [Co(NH ) ]Cl orange-yellow

(b) 3 3 3 5 2CoCl .5NH [Co(NH ) CI]Cl Violet

(c) 3 3 3 4 2CoCl .4NH [Co(NH ) Cl ]Cl Violet

(d) 3 3 3 3 3CoCl .3NH [Co(NH ) Cl ] green limx→∞

Postulates

• There are two types of valencies shown by central metal, metal ion in a compound:

(a) primary (b) secondary

Primary valency corresponds to oxidation number while a secondary valency corresponds to coordination number

• In the complex 3 6 3[ ( ) ]Co NH Cl , primary valency of three and secondary valency six.

• Primary valency is satisfied by negative ions (and while secondary valency is satisfied by ligands which be neutral , and as well as negative (positive in rare case).

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- In III3 6 3[Co (NH ) ]Cl , six 3NH ligands secondary valencies while three -Cl satisfy

valencies.

- In II3 4 2 2 4[Co (NH ) (H O) ]SO , four 3NH and two (ligands) satisfy secondary and one

2-4SO primary valencies.

• Ligands/ions attached be secondary valency do not while ions attached by primary valency ionize.

+ 4-4 6 6K [Fe(CN) ] 4K +[Fe(CN) ]�

4+ -3 6 4 3 6[Pt(NH ) ]Cl [Pt(NH ) ] +4Cl�

• The secondary valencies are directional and are responsible for the isomerism in complexes.

• Primary valency in the complex is represented by ……. While secondary valency by……… • In all cases, metal or metal ion should satisfy primary and secondary valencies both.

Some negative ions may satisfy primary as well as secondary valencies (dual nature). In the complex, they are represented as ……..

Werner’s Representation Consider the case of 3 3CoCl . NHx

Where maximum value of =C.N. of Co (III)=6x

And minimum value of =C.N. - O.N. =3x

Various structures are summarized in Table

Table2.8: Werner’s Coordination Compounds

Werner Modern notation

Ionisation Secondary valency satisfied by

Primary valency satisfied by

(A) 3 3Co.Cl .6NH 3 6 3[Co(NH ) ]Cl 3+ -3 6[Co(NH ) ] +3Cl six 3(NH ) three -(Cl )

(B) 3 3Co.Cl .5NH 3 5 2[Co(NH ) Cl]Cl

2+ -3 5[Co(NH ) Cl] +2Cl five 3(NH ) and

one -(Cl )

three -(Cl )including

one -(Cl )with dual nature

(C) 3 3Co.Cl .4NH 3 4 2[Co(NH ) Cl ]Cl

+ -3 4 2[Co(NH ) Cl ] +Cl four 3(NH ) and

two -(Cl )

three -(Cl )including

two -(Cl )with dual nature

(D) 3 3Co.Cl .3NH 3 3 3[Co(NH ) Cl ] 3 3 3[Co(NH ) Cl ]

(single species)

three 3(NH )

and three -(Cl )

three -(Cl )all with dual nature

• It is clear that conduction of the complex which depends on the number of ions will be in

order.

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D<C<B<A • They are represented as

[Only -Cl ions joined by (……) will ionise]

1.11 Bonding in Complex

Valence Bond Theory (VBT) One way to think about bonds between a metal ion and its ligands is based on coordinate covalency. The central ion furnishes the orbitals and the ligands furnish electron pairs. Although this view is now considered inadequate, it does account for the observed coordination number of the central ion, and it also describes the geometric structure of the complex ion. Let us study some complex to see how the valence bond theory explains their properties. Take example of 3+

2 6[Fe(H O )] . The complex is known to be paramagnetic, with a magnetic

moment corresponding to five unpaired electrons. This can be described if we follow.

Electronic configuration of 26Fe

Electronic configuration of 3+Fe (it is formed by loss of the 4s electrons and one of the 3d electrons) To account for the experimentally observed fact that 3+

2 6[Fe(H O )] has five unpaired

electrons, each 3d-orbital is assumed to have one unpaired electrons in the complex. The

vacant 2 2 2x z4d and 4d

y−orbitals are hybridized with the vacant 4s-and 4p-orbitals.

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Each of the six 2H O ligands donates one electron pair into one of the six 3 2sp d -orbitals,

forming six coordinate-covalent bonds. These donated electrons 2(from O of H O ligands)

have been represented as "x" rather than" "↑ . These are the only bonding electrons; none of the 3d-electron of iron is involved in bonding. A set of outer d orbitals is used in hybridization, and hence 3+

2 6[Fe(H O) ] is called an outer

orbitals complex.

In deciding whether the hybridization at the central metal ion of octahedral complex is

3 2sp d (outer orbital) or 2d sp (inner orbital) we must know the result of magnetic

measurements. These indicate the number of unpaired electrons (N). Magnetic moment is given by

µ= N(N+2) BM

(B.M is Bohr Magneton) In 3- 3+

6[Fe(CN) ] , Fe is involved. But magnetic measurements indicate only one unpaired

electron i.e iron atom. The 2 2 2x -y z3d and 3d orbitals (rather than those in 4d-shell) are

involved in 2 3d sp hybridization. All but one of the non-bonding 3d-electrons of 3+Fe are

paired in this complex.

Only inner d-orbitals are used in hybridization, hence 3-

6[Fe(CN) ] is called an inner orbital

complex.

Example- Explain the paramagnetic behavior of 2+3 6[Co(NH ) ]

Solution- Oxidation state of Co=+2

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When hybridization occurs, one of the 3d-electrons is excited 5s-orbital. The suggested bonding for 2+

3 6[Co(NH ) ] is consistent with that this complex is easily

oxidized to 3+3 6[Co(NH ) ] , an extremely complex ion.

Since there exists one unpaired electron, this complex ion is paramagnetic. Magnetic moment is given by

µ= N(N+2) BM= 3 BM

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Table 2.9 : Bonding and Hybridization in some Octahedral Complex

1.12 Crystal Field Theory

The valence-bond theory has some short comings.

• It does not provide insight into the origin of the characteristic colour of the complex ions. • It does not distinguish between weak and strong ligands. • It does not give quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds. • There is no quantitative interpretation of magnetic data. • It does not explain why some complex are inner-orbital complex and some complexes are

outer-orbital.

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Most of the shortcomings of VBT of coordination compounds were removed in Crystal Field Theory (CFT). It attributes partial covalent character and partial ionic character to bonds.

Complex with C.N. 6 (Octahedral Crystal Field) • Five 2 2 2xy yz xz x -y z

(d ,d ,d ,d and d ) are of equal energy and thus are degenerate.

• The 2 2 2x -y zd and d orbitals are directed along a set of mutually perpendicular x,y and z

axes. As a group, these orbitals are called ge orbitals.

• The xy yz xzd ,d and d orbitals lie between the axes and collectively called 2gt orbitals.

• The ligand donor atoms approach the metal ion along the axes to form octahedral complexes. Crystal field theory proposes that the approach of the six donor atoms along the axes sets up an electric field (crystal field). Electrons on the ligand repel electrons in

ge orbitals on the metal ion more strongly than they repel those in 2gt orbitals and splits

them into two sets, the ge set at higher energy and the 2gt set at lower energy.

The energy separation between the two sets is called Octahedral∆ or oct∆ . It is proportional to

the crystal field of the strength of the ligands.

• The d-electrons on a metal ion occupy the 2gt set in preference to the higher energy, ge

set. Electrons that occupy the ge orbitals are strongly repelled by the relatively close

approach of ligands. Electron in ge set tends to destabilise octahedral complexes.

• Based on valence bond theory, 3-6[CoF ] is a paramagnetic outer orbital complex while

3+3 6[Co(NH ) ] is a diamagnetic inner orbital complex. Both contain 6 3+d Co ions (four

unpaired electrons) in its 3d orbitals.

Magnetic measurements indicate that 3-

6[CoF ] also has four unpaired electrons per ion.

Thus, there must be four electrons in 2gt orbitals and two in ge orbitals.

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On the other hand, 3+

3 6[Co(NH ) ] is diamagnetic, thus all the six d-electrons must be

paired in the 2gt orbitals.

The difference in configuration between 3-

6[CoF ] and 3+3 6[Co(NH ) ] is due to the relative

magnitude of the crystal field splitting, oct∆ caused by the different crystal field strengths

of - 3F and NH . The 3NH molecule interacts with vacant metal orbitals more strongly than

the - F ion does.

3+ 3-oct 3 6 oct 6∆ for [Co(NH ) ] ∆ for [CoF ]>

The crystal field splitting for 3-6[CoF ] complex ions is very small and thus, an energetically

more favourable condition may arise if two electrons remain unpaired in the antibonding

ge orbitals. After all d-orbitals are half-filled, additional electrons will pair with electrons in

2gt set. This is the case for 3-6[CoF ] , which is called a high spin complex.

For 3-6[CoF ] : -F is a weak field ligand, So oct∆ (electron pairing energy), thus, high spin

complex. A high spin complex in crystal field terminologist corresponds to an outer orbital complex in VBT. For 3+

3 6[Co(NH ) ] : 3NH is a strong field ligand so that oct∆ >P, thus low spin complex

A low spin complex corresponds to an inner orbital complex. • Low spin configurations exist only for octahedral complexes having metal ions with

4 5 6d , d , d and d7configurations. For 1 3 8 10d -d and d -d ion configuration is designated as

high spin. All of possibilities have been shown in Table.

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Table 3.0: High and Low Spin Octahedral Configurations

Complex with C.N.4 (Tetrahedral Crystal Field) In tetrahedral complex formation, the d-orbital splitting is inverted and smaller as compared to the octahedral field splitting. If ∆ is the magnitude of energy separation in tetrahedral crystal field, then for the same metal, the same ligands and metal-ligand bond distance,

t oct

4∆ =- ∆

9

In such cases, low spin configuration are rarely observed.

• The magnitude of oct∆ or t∆ , the orbitals splitting energy is decided by

Nature of the ligand: Which have been arranged in increasing field strength as:

- - - - - 2- - -2 2 4 3 2I <Br <Cl <F <OH <H O<C O <NH <en<NO <CN

- Geometry of the complex: t∆ value (of tetrahedral complex) is about 50%

compared to oct∆ value (of octahedral complex)

- Nature of the metal ion: going from Cr to W (through MO), oct∆ increases 50% in

each series change (i.e., 3d to 4d to 5d). Thus, there is greater tendency of the second

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(4d) and third (5d) transition series to be low spin as compared to first (3d) transition series.

- Oxidation state of the metal ion

3+ 2+∆(Fe )>∆(Fe )

2+ +∆(Cu )>∆(Cu )

Thus, greater the ionic charge on the central metal ion, the greater the value of ∆ .

1.13 Colour and the Spectrochemical Series. A substance appears coloured because it absorbs length that corresponds to one or more of the wavelength in the visible region of the electromagnetic spectrum (4000 to 7000

�)

and transmits or reflects the other wavelength. A combination of all wavelengths in the visible region is called “white light”, sunlight is an example. The obsence of all wavelength in the visible region is blackness. Most transition metal compounds are coloured. In transition metal compounds, the d-orbitals in any one energy level of the metal are not degenerate. They are often split into two sets of orbitals separated by energies, oct∆ , that correspond to wavelength of light in the visible

region. The absorption of visible light causes electronic transition between orbitals in these sets. Colour of some transition metal nitrates in aqueous solutions are given below: Table 3.1: Colour of some Transition Metal ions

Transition Metal Ion Colour of Aq. Solution 3+Cr Deep blue

2+Mn Pale pink 2+Fe Pale green 3+Fe Orchid 2+Co Pink 2+Ni Green 2+Cu Blue

One transition of a high spin octahedral Co(III) complex is represented below.

The frequency (�), and thus the wavelength and colour, of the light absorbed are related

to oct∆ . This, in turn, depends on the crystal-field strength of the ligands. Hence, the colour

and the visible absorption spectra of transition metal complexes, as well as their magnetic properties, provided information about the strengths of the ligand-metal interactions. Table gives the relationship among colours absorbed and colours transmitted or reflected in the visible region. The spectral colour is the colour associated with the wavelengths of the light absorbed by the sample. The complimentary colour is the colour associated with the wavelengths that are not absorbed by the sample, and is seen when the spectral colour is removed from white light. Complimentary colour of orange is spectral colour blue and vice-versa and is represented in the form of a wheel (to remember)

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Table 3.2 : Complimentary Colour

Wavelength Absorbed(�) Spectral Colour (Colour

absorbed) Complimentary Colour (Colour observed)

4100 violet lemon yellow 4300 indigo yellow 4800 blue orange 5000 blue-green red 5300 green purple 5600 lemon yellow violet 5800 yellow indigo 6100 orange blue 6800 red blue-green

Fig:- Spectral Colour and Complimentary colour are opposite to each other • By study of visible spectra of many complexes, it is possible to arrange common ligands

in order of increasing crystal field strength. - - - - - 2- - -

2 2 4 3 2I <Br <Cl <F <OH <H O<C O <NH <en<NO <CN

This arrangement is called the spectrochemical series. • Strong field ligands, such as -CN , usually produce low spin complexes, where possible,

and large crystal field-splittings. • Weak field ligands, such as -Cl , usually produce high spin complex and small crystal

field- splittings. Low spin complexes usually absorb higher-energy (short-wavelength) light than do high spin complexes.

• In 3+3 6[Cr(NH ) ] , the Cr(III) is bonded to six 3NH ligands which produce a relatively high

value of oct∆ . This absorbs light in the blue and violet regions of visible light and thus

orange-yellow complimentary colour is observed. • 2H O is a weaker field ligand than 3NH ,hence,

3+ 3+oct 2 6 oct 3 6∆ ([Cr(H O) ] )<∆ ([Cr(NH ) ] ) And thus 3+

2 6[Cr(H O) ] absorbs lower-energy light

and appears blue (complimentary colour).

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Limitations: Crystal Field Stabilisation Energy

Electrons in the 2gt orbitals of an octahedral complex are lower in energy, while electrons

in ge orbitals are higher in energy, than they would be in a spherical field. The lower the

total energy of a system, the more stable it is. The crystal field stabilization energy (CFSE) of a complex is a measure of the net energy of stabilization of a metal ions electron’s CFSE is zero for weak field (high spin)

octahedral 0 5 10d , d and d complexes.

Ion nd

CFSE=0

2+Ca 0d 2+Mn 5d

2+Zn 10d Example-Draw a crystal field orbital energy level diagram, and predict the number of unpaired electrons for each of the following complex. (a) 3+

3[Cr(en) ] (b) 3-6[Mn(CN) ] (c) 2+

2 6[Co(H O) ]

Solution- (a) 3+ 3Cr ([Ar]3d ) has three unpaired electrons. In the complex, they occupy the

lower set of d-orbitals.

(b) 3+ 4Mn ([Ar]3d ) Can have a high-spin or low-spin configuration. Because -CN is a strong-

field ligand, all four d-electrons go into the lower –energy d-orbitals. The complex is low-spin with two unpaired electrons.

(c) 3+ 7Co ([Ar]3d )Has a high-spin configuration with three unpaired electrons because 2H Ois a weak-field ligand. In the following orbital energy level diagram, the relative values of a crystal field splitting ∆are in accordance with the position of the ligands in the spectrochemical series

-2(H O<en<CN ) .

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1.14 Stability Constants and Stability of Coordination Compounds

Consider the stepwise formation of the complex 4ML from metal ion 4+M (Lewis acid) and

ligand -L (Lewis base). Let 2 3 4k ,k and k be stepwise formation constants and 4β ( )fk= is

overall formation constant. Thus,

4 1 2 3 4β k k k k=

4 1 2 3 4log log log log logk k k kβ = + + +

If the complex is n, and nML β the overall formation constant, then

nM nL ML+ � (charges omitred)

1 2 3 4

[ ]...

[ ][ ]n

n nn

MLk k k k k

M Lβ = =

n 1 2 3log log log log ... log nk k k kβ = + + + +

• ,nβ The stability constant, is related to thermodynamic stability when the system has

reached equilibrium. Most of the measurements have been made from aqueous solutions, which means that when complex is formed stronger ligand L(say CN − , en)

displaces weaker ligand 2H O of the aqua complex of the metal ion.

2 n n 2M(H O) +nL ML +nH O�

• For a given ion and ligand, greater the charge on the metal ion, greater the stability. Thus, stability of

III 3- II 4-6 6[Fe (CN) ] >[Fe (CN) ]

• For the divalent ions of the first row transition elements (3d-series), thus stabilities vary in the Irving-Williams order:

II II II II II IIMn <Fe <Co <Ni <Cu <Zn • Ligands containing N, O or F donor atoms form most stable coordination compounds

when metal ions are. -groups 1 and 2 elements, the inner transitions elements and transition elements of

groups 3 to 6 (Sc, T, V, Cr) called ‘Class a’ acceptors.

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-Transition elements-Rh, Pd, Ag, Ir, Au and Hg having relatively completely filled d-orbitals called ‘class b’ acceptors.

- The stability also depends on the formation of chelate rings and stabilization due to chelation is called the chelation effect. This effect is maximum for the five and six- membered rings. • If a multidentate ligand happens to be cyclic and there is no unfavourable steric effect, a further increase in stability occurs. Thus it is termed as macrocyclic effect. 1.15 Kinetic Consideration of Ligand Displacement

When we add 3NH (aq) to a solution containing 2+Cu , we observe a change in colour from

pale blue to deep blue. The reaction involves 3NH molecules displacing 2H O molecule as

ligands.

2+ 2+

2 4 3 3 4 2[Cu(H O) ] + 4NH [Cu(NH ) ] +4H O

(pale blue) (very deep blue)

→

This reaction occurs very rapidly as rapidly as the two reactions can be brought together. The addition of HCl(aq) to an aqueous solution of 2+Cu produces an immediate colour change from pale blue to green, or even yellow if the HCl(aq) is sufficiently concentrated.

2+ - 2+

2 4 4 2[Cu(H O) ] +4Cl [CuCl ] +4H O

(pale blue) yellow

→

Complex ions in which ligands can be interchanged rapidly are said to be labile. 2-

4[CuCl ] , 2+ 2-3 4 4[Cu(NH ) ] and [CuCl ] are all labile.

In freshly Prepared +3 2 4 2CrCl (aq),[Cr(H O) Cl ] produces a green colour, but colour

gradually turns to violet. This colour change results from the very slow exchange of 2H O for

a -Cl ligands. A complex ion that exchanges ligands slowly is said to be non-labile or inert. In general, complex ions of the first transition series, except for those Cr(III) and Co(III)are kinetically labile. Those of the second and third transition series are generally kinetically inert.

1.16 Applications of Coordination Chemistry • The photographic “fixing” process removes unreacted AgBr which forms soluble complex with sodium thiosulphate.

2- 3- -2 3 2 3AgBr(s)+2S O [Ag(S O )] +Br→

• In qualitative analysis of group I, there is formation of white precipitate of AgCl, Hg2Cl2, and PbCl2. PbCl2 is soluble in hot water. AgCl can be removed as soluble [Ag(NH3)2]Cl on adding NH3 (aq):

3 3 2( ) 2 ( ) [( ) ]AgCl s NH aq Ag NH Cl+ ˆ ˆ†‡ ˆ ˆ

In group II, Cd2+ can be precipitated as CdS in presence of Cu2+. By treating the solution of Cu2+ and Cd2+ with an excess of CN-, Cu2+ forms a stable complex in a redox reaction:

2 34 2 22 10 2[ ( ) ] ( )Cu CN Cu CN C N g+ - -+ ® +

While 2+Cd forms less stable complex.

2+ - 2-4Cd +4CN [Cd(CN) ]�

soluble

stable

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Unstable When 2H S gas is passed, 2-

4[Cd(CN) ] is precipitated as CdS without affecting 3-4[Cu(CN) ]

complex.

• Electrolyte solutions used in commercial electroplating are quite complex. A number of metal such as Cu, Ag and Au are generally plated from solutions of their cyano complex ions. In the electrolysis reactions below the object to be plated is made the cathode and the piece of copper metals is the anode.

Anode: - 3-4Cu+4CN [Cu(CN) ] e→ + −

Cathode: 3- - -4[Cu(CN) ] +e Cu+4CN→

• The net change simply involves the transfer of Cu metal from the anode to the cathode through the formation, migration and decomposition of complex ion 3-

4[Cu(CN) ] .

• Purification of water is also through complex formation. One method of water treatment involves chelation. Among the chelating agents widely employed are the salts of

EDTA, usually the sodium salt. The formation constants of 2-[Ca(EDTA)] and

2-[Mg(EDTA)] are very large. Hence, on treating water with sodium salt of EDTA,

concentration of 2+Ca and 2+Mg can be reduced to the point that these ions will not

precipitate with soap or detergent.

2+ 4- 2-Ca +(EDTA) [Ca(EDTA)]�

2+ 4- 2- 8fMg +(EDTA) [Mg(EDTA)] 4 10k = �

• Chelation with EDTA can also be used in the treatment of metal poisoning.If a person

suffering from lesd poisoning is fed 2-[Ca(EDTA)] , the following exchange occurs.

2+ 2- 2- 2+Pb +[Ca(EDTA)] [Pb(EDTA)] +Ca→

Because fk of 2- 18[Pb(EDTA)] is 1×10 and is thus more stable than 2-[Ca(EDTA)] . The

lead complex is excreted by the body and the 2+Ca remains as the body nutrient. • The extraction of silver and gold involves the formation of cyanide complex.

- - -2 2 24Au( )+8CN ( )+O ( )+2H O( ) 4OH ( )+4[Au(CN) ] ( )s aq g l aq aq→

22 42[Au(CN) ] ( ) Zn( ) [Zn(CN) ] ( ) 2Au( )aq s aq s− −+ → +

Mond’s process involves formation of carbonyl complex of nickel. 4Ni( ) 4CO( ) Ni(CO) ( )s g g+ →

• Traces of 2+Ni salt can be detected by dimethyl glyoxime when cherry red precipitate of nickel-dimethyl glyoximate (chelate) is formed.

C is-diammine dichloroplatinum (II), 3 2 2[Pt(NH ) Cl ] is

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One of a number of platinum coordination compounds used in the treatment of cancer. Commonly known as cisplatin, this compound has the ability to block the uncontrolled division of cancerous cells that results in the growth of tumours. Recent studies show that cisplatin can cause serious side effects, including severe kidney damage. It is being replaced by dinuclear platinum complex.

• Organometallic compound must contain, at least one metal-carbon bond. 4Ni(CO) is also

considered organometallic compound, though CO is not considered to be an organic compound. The suffix ‘metallic’ often includes metalloids like boron, silicon, arsenic and tellurium as well as true metals. Metal-cyanides, though possessing an M-C bond are not included in organometallics. • Main group organometallics E.C. Frankland was the first chemist to synthesise an organometallic compound. The s-and p-block organometallics are named according to substituent names used in organic chemistry. 3CH Li methyl lithium

3 3B(CH ) trimethyl boron

3 4Si(CH ) tetramethyl silane

3 3As(CH ) trimethyl arsane

The oxidation number of the metallic element in an organometallic compound is based on the organic part being considered to be anionic. Thus, 3CH group in 3 2Zn(CH ) has (-1)

charge. • d-and f-block organometallics W.C. Zeise was the first to prepare organometallic compound of d-block elements.

Example are - trichloroethene platinum (II): -

3 2 4[PtCl (C H )]

- tetracarbonyl nickel (0): 4Ni(CO)

- bis(cyclopentadienyl) iron (II): 5 4 2Fe(C H ) , also called ferrocene.

• In metal carbonyls, CO as a ligand binds itself to metal atom through the carbon atom. It is a weak donor (a weak Lewis base) and forms a weak σ -bond to the central atom. CO is also an acceptor (Lewis acid) ligand and forms πbond to the metal. This characteristic property of back bonding stabilizes the metal –ligand interaction.

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1.17 Organometallic Compounds

These are the compounds which contain one or more metal-carbon bond in their molecules. They can be classified into different categories.

(a) Sigma bonded compounds. In such compounds, the metal-carbon bond is sigma bond. Some examples are:

R-MgX (Grignard’s reagent), R2CD (Dialkyl cadmium)

R2ZN (Dialkyl zinc); Al2(CH3)6 etc.

Al2(CH3)6 is a dimer of Al(CH3)3 in which two of the methyl groups are held by the Al atoms by three centre bonds whereas the other four CH3 groups are held by single covalent bonds.

(b) Pi bonded complexes. In these compounds, molecules/ions containing π bonds acts as ligands.

They contain sigma bond formed by axial overlap of π M.O. of the ligand with some vacant orbital of the metal. At the same time a π bond is formed by side-wise overlap of some filled orbital of metal atom with vacant π∗ (antibonding) M.O. of the ligand. This is referred to as back π bonding and such ligands are called π acid ligands.

The number of carbon atoms simultaneously bound to central metal atom by donation of electron density from π cloud is indicated in the power of η i.e., ηx.

Some examples:

22 4 3[ ( ) ]

( )

K Pt n C H Cl

Potassiumtrichloro

ethylene platinate II

−−

55 5 2( )

( )( )

Fe C H

Bis Cyclopentadiencyl

iron II

η −

55 5 2( )

( )( )

Os C H

Bis Cyclopentadiencyl

osmium II

η −

66 6 2( )

( )Cr C H

Dibenzenechromium o

η −

(c) Metal carbonyls. In these compounds CO molecule acts as ligand. In metal carbonyls also the metal-carbon bond posses both sigma and π-character as explained earlier. Some examples of metal carbonyls are:

Ni(CO)4 Cr(CO)6

Tetracarbonylnickel (o) Hexacarbonylchromium (o)

Fe(CO)5 Mn2(CO)10

Pentacarbonyl iron (o) Decacarbonyldimaganese (o)

(It contains Mn-Mn bond also)

Co4(CO)12 Co2(CO)8

Dodecacarbonyltetracobalt (o) Octacarbonyldicobalt (o)

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Preparation of Metal Carbonyls

By oxidative addition of organic halides to metals,

RX + Mg →EtherRMgX

RX + 2M → R – M + MX (M = Li, K etc.)

By reaction of other organometallics with metal halides,

4 2 5 2 5 44 ( ) 4 ( )PbCl C H MgBr C H Pb Mg Br Cl+ → + 1.18 Bonding in organometallic compounds:

The nature and structure of the group (ligand) forming organometallic compounds determine the type of bonding in them. (a)Bonding of alkenes to a transition metal atom: The structure of an alkene is shown

in fig.It has filled π-orbital and unfilled anti bonding π*-orbital. The bonding of alkene molecule to transition metal atoms takes place in two steps.

(i) Formation of sigma bond: It takes place when sigma donation occurs from filled π-orbital of the alkene molecule into a vacant orbital on the metal atom.

(ii) Formation of π-bond: After the formation of sigma bond,π-bond donation occurs from a filled metal d-orbital on the metal atom into the vacant anti bonding (π*) orbital of alkene molecule as shown below.Thus, the complete bonding of alkene molecule to a transition metal through the above steps can be shown in figure.

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(b) Bonding of carbon monoxide with transition metals: The bonding in transition metal carbonyls also takes place in two steps:

(i) Formation of sigma bond: In this step, donation of a lone pair of electron on carbon atom takes place into a vacant d-orbital on the transition metal atom and sigma bond is formed.

(ii) Formation of π-bond: After the formation of a sigma bond, π-back donation of electrons occurs from a filled d-orbital into a vacant anti-bonding (π*) orbital of carbon monoxide,CO.The complete metal CO compound can be seen as:

1.19 Importance and Applications of Co-ordination Compounds

The coordination compounds are of great importance.These compounds are widely present in the mineral, plant and animal worlds and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems; industry and medicine.These are described below:

• Co ordination compounds find use in many quanlitative and quantitative chemical analysis. The familiar colour reactions given by metal ions with a number of ligands(especially chelating ligands), as a result of formation of coordination entities,form the basis for their detection and estimation by classical and instrumental methods of analysis.Examples of such reagents include EDTA, DMG(dimethylgloxime), α-nitroso-β-naphthol, cupron etc.

• Hardness of water is estimated by simple titration with Na2EDTA.The Ca2+ and Mg2+ ions form stable complexes with EDTA.The selective estimation of these ions can be done due to difference in the stability constants of calcium and magnesium complexes.

• Some important extraction processes of metals, like those of silver and gold, make use of complex formation.Gold, for example, combines with cyanide in the presence of oxygen and water to form the coordination entity [Au(CN)2]

- in aqueous solution.Gold can be seprated in metallic form from this solution by the addition of zinc .

• Similarly, purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds.For example, impure nickel is converted to [Ni(CO)4],which is decomposed to yield pure nickel.

• Coordination compounds are of great importance in biological systems.The pigment responsible for photosyntyhesis, chlorophyll, is a coordination compound of magnesium.Haemoglobin, the red pigment of blood which acts as oxygen carrier is a coordination compound of iron.Vitamin B12

, cyanocobalamine, the antipernicious

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anaemia factor, is a coordination compound of cobalt.Among biological importance with coordinated metal ions are the enzymes like, carboxypeptidase A and carbonic anhydrase(catalysis of biological systems).

• Coordination compounds are used as catalysts for various industrial processes.Examples include rhodium complex, [(Ph3P)3 RhCl], a Wilkinson catalyst, is used for the hydrogenation of alkenes.

• Articles can be electroplated with silver and gold much more smoothly and evenly from solutions of the complexes,[Ag(CN)2]

- and [Au(CN)2]- than from a solution of

simple metal ions. • In black and white photography, the developed film is mixed by washing with hypo

solution which dissolves the undecomposed AgBr to form a complex ion,[Ag(S2O3)2]3-.

• There is growing interest in the use of chelate therapy in medicinal chemistry. An example is the treatment of problems caused by the presence of metals in toxic proportion in plant/animal systems.Thus, excess of copper and iron are removed by the chelating ligands D-penicillamine and desferrioxime B via the formation of coordination compounds.EDTA is used in the treatement of lead poisoning.Some coordination compounds of platinum effectively inhibit the growth of tumours.Examples are: cis-platin and related compounds.

1.20 Importance and Applications of Organometallics

Organometallics are of immense importance.progress in this area has introduced new reagents and catalysts for synthesis.Some of the applications of organometallic compounds are given below:

(I) Homogeneous catalysis.A large number of reactions in solutions are catalysed by organometallic compounds or intermediates derived from transistion metal complexes.For example Wilkinson’s catalyst,[RhCl(PPh3)3] is used as a catalyst in the hydrogenation of alkenes, as already discussed above.

(II) Heterogeneous catalysis: Organometallic compounds can also be used as heterogeneous catalysts e.g., trialkyl aluminium mixed with transition metal halide, usually titanium trichloride or tetrachloride (Ziegler-Nata type catalyst) is used as a catalyst for the low temperature polymerization of alkenes.

(III) Purification of metals: As already discussed, impure metals are first converted into their carbonyls followed by their decomposition to get the pure metal.

(IV) Organic synthesis: They are widely used in synthesis of various types of organic compounds, e.g., organolithium and organomagnesium compounds are commonly used for the synthesis of different types of organic compounds.

(V) Medicines: A number of organo arsenic compounds are used as main remedy for syphilis. The modern surgery makes use of silicon rubbers for spare parts in the body.

(VI) Agriculture: To prevent the infection of young plants, the seeds are treated with or-ganometallics such as ethyl mercury chloride.

(VII) Future expectations: It is expected that the organometallic compounds will play an important role in the production of pharmaceuticals, flavours, fragrances and semi-conductors for the electronic industry.