SYNTHESIS, SPECTROSCOPIC, ELECTRONIC, AND MAGNETIC PROPERTIES OF SOME 3D METAL COMPLEXES

C. C. PALAD INSTITUTE OF CHEMISTRY, COLLEGE OF SCIENCEUNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY, PHILIPPINESOCTOBER 21, 23, 28, AND 30H. SALVADOR, D. DE CHAVEZ

1. Rationalize the synthetic design of each of the coordinationcompound synthesized. Write down chemical equations with each relevantstep. Why is a particular metal salt for cobalt used in each of thecobalt compound synthesized?

Potassium trioxalatochromate(III) trihydrate, K3[Cr(C2O4)3] 3H∙ 2O

K2Cr2O7 + H2C2O4·2H2O K[Cr(C2O4)2(OH2)2] + H2O + CO2 (1)

When potassium dichromate was reacted with oxalic acid, the reactionwas violent and effervesced carbon dioxide gas. A redox reactionoccurred where the metal Cr6+ from the dichromate ion is reduced toCr3+. This Cr3+ then forms the octahedral oxalato complex.

K[Cr(C2O4)2(OH2)2] + K2C2O4 K3[Cr(C2O4)3] + H2O (2)

Water ligands were displaced by the oxalate ions. This process wasaccelerated by applying heat.

Potassium trioxalatoferrate (III) trihydrate, K3[Fe(C2O4)3] 3H∙ 2O

Double displacement reaction occurs.

3K2C2O4•H2O + FeCl3•6H2O → K3[Fe(C2O4)3]•3H2O + 3KCl + 6H2O (3)

Potassium trioxalatocobaltate(III) trihydrate, K3[Co(C2O4)3] 3H∙ 2O

The lead dioxide oxidized the Co2+ to Co3+.

CoCO3 + 3K2C2O4•2H2O → [Co(C2O4)3]K2 + 2CO2 (4)[Co(C2O4)3]K2 + PbO2 → [Co(C2O4)3]K3•3H2O (5)

Tris(ethylenediamine)cobalt(III) chloride, [Co(en)3]Cl3

HCl reacts with en via an acid-base reaction, forming the complexcation [Co(en)3]2+

CoCl2•H2O + 3en•2HCl → [Co(en)3]Cl2 +6H2O + 6HCl (6)

The Co2+ reacted with hydrogen peroxide oxidized it to become Co3+, asshown in the reaction below:[Co(en)3]Cl2 + H2O2 → [Co(en)3]Cl3 + 2OH- (7)

Hexaamminecobalt(III) chloride, [Co(NH3)6]Cl3

CoCl2·H2O + NH4Cl + 5NH3 + H2O2 [Co(NH3)6]Cl3 + H2O + ½O2 + H3O+ (8)

Co2+ was then again oxidized to Co3+ by hydrogen peroxide, which is whathappened in Equation 7. Hydration is prevented by adding excessammonia as the complex is more stable in ammonia solvent.

Pentaamminechlorocobalt (III) chloride, [Co(NH3)5Cl]Cl2

The previous complex was used to synthesize this linkage isomer. Thebrown slurry that was formed was actually the oxidation of Co2+ to Co3+

facilitated by hydrogen peroxide.

[Co(NH3)6]Cl3 + HCl [Co(NH3)5Cl]Cl2 + NH4Cl (9)

HCl provided the Cl ligand that formed the linkage isomer; the H+

protonated the NH3 for it to act as a better leaving group. This

reaction was carried out at a high temperature such that the reactionwould be accelerated.

Pentaamminenitritocobalt (III) chloride, [Co(NH3)5ONO]Cl2

Complex 6b was formed from Complex 6a. The reaction proceeded in twosteps:

[Co(NH3)5Cl]Cl2 + H2O [Co(NH3)5OH2]Cl3

(10)

[Co(NH3)5OH2]Cl3 + NaNO2 [Co(NH3)5ONO]Cl2 + H2O + NaCl (11)

Pentaamminenitrocobalt(III) chloride, [Co(NH3)5NO2]Cl2

[Co(NH3)5ONO]Cl2 [Co(NH3)5NO2]Cl2 (12)

6b is the thermodynamic product in this reaction.

2. Why are we preparing Co(III) compounds though starting from Co(II)salts?

Co(III) complexes are more stable than Co(II) complexes. The Co(II)salts are oxidized to Co(III) by addition of hydrogen peroxide, and isthen reacted immediately to add ligands.

3. Consider the metal ions in which the color of the solution issimilar to those of the hydrated ion. What does the similarity incolor suggest regarding the dominant species of the ion upondissolution?

Metal ions that retain their cationic form compared to the hydratedspecies do not have a difference in color.

4. In which compounds are the colors of the solution markedlydifferent from that of the corresponding hydrated ion? What could bean explanation for this? Provide a chemical equation to support youranswer. Give your basis as to why the reaction proceeds accordingly.

Hydrated cobalt complex is pink. Meanwhile, solid cobalt complexesthat have been synthesized in this experiment have various colors(e.g. K3[Co(C2O4)3]∙3H2O, [Co(en)3]Cl3, and [Co(NH3)6]Cl3 are dark blue-green, orange, and light orange in color when in crystal form).Substitution of other stronger or weaker field ligands that displacethe water in the metal center accounts for the change of ∆o

.

5. For the linkage isomerism, account for the kinetically controlledproduct.

Complex 6c is the thermodynamic product, while Complex 6b is thekinetically controlled product.

6. Differentiate the point groups of the two linkage isomers prepared.How will their difference account for the IR spectral difference inthese complexes?

[Co(NH3)5ONO]Cl2 has the point group C1 while compound its linkageisomer [Co(NH3)5NO2]Cl2, has a C2v point group. The difference in theirpoint groups account for the IR spectral difference since thestretching would change, as symmetric and asymmetric IR stretches canbe found in the latter, while Co(NH3)5ONO has one since it is a C1

pointgroup.

7-8. Among the 1st row transition metal species, only zinc ion iscolorless. Why is this so? Zinc ion does not usually exhibit the samebehavior as the rest of the transition metals.

The zinc metal, unlike the first row d-block elements such asScandium, Titanium, Vanadium, and Chromium, etc. has fully filled dorbitals. This is the reason why it does not exhibit the same behavior(unreactive and stable) as the rest of the transition metals. The Znhas an electronic configuration of [Ar]3d104s2, the Zn metal loses itsouter 4s2 electrons, which still remains the 3d orbital fully filled,to form the colorless Zn2+ which does not undergo d-d transitionsunlike the other transition metals.

9. Using Tanabe-Sugano diagrams, assign transitions with the observedpeaks in the spectra.

Complex λmax, nm Energy (∆o), cm-

1Transitions

1 (d3)572 18210 4T1(G) ← 4T1(F)418 23750 4T1(F) ← 4T2

213.5 38501 4T2 ← 4A2

2 (d5, highspin)

- - -

3 (d6, highspin)

251 - -238 - 5E ← 5T2

4 (d6, lowspin)

400.5 22585 1T1←1A1

326.5 30238 1T2←1T1

209.5 43496 1E←1T2

287 40123 1A2(P)←1E209.5 43496 1A2(F)←1A2(P)

5* N/A N/A N/A

6a (d6, lowspin)

423 18194 5E ← 5T2

359.5 27810 1T1←1A1

649.5 15140 1T2←1T1

423 23601 1E←1T2

330.5 30560 1A2(P)←1E

6b (d6, lowspin)*

- - 5E ← 5T2

- - 1T1←1A1

- - 1T2←1T1

- - 1E←1T2

- - 1A2(P)←1E

6c (d6, lowspin)

458 21834.06 5E ← 5T2

298 33557.05 1T1←1A1

602.5 16597.51 1T2←1T1

398 25125.63 1E←1T2

288.5 34662.05 1A2(P)←1E* - no data obtained

10. Are there any weak bands with extinction coefficients of about 1%of normal? If so, identify the transition responsible for these weakbands. Why are these bands so weak?

Yes, there are such weak bands that exist. This is due to the Laporteand d-d forbidden transitions. The allowed transition is only from u->u and g->g.

11. Discuss how changing the ligand, while keeping the same metal,affects the value of ∆o. Cite specific examples and values from yourdata.

The effect of changing a ligand heavily relies on whether the ligandthat would be used to replace an existing one in the complex is eithera strong or weak field ligand. A weak field ligand would cause greaterenergy difference between the d-orbitals and thus allow pairing of thelow spins first. Meanwhile, a strong field ligand does otherwise.According to Crystal Field Theory, a metal and a ligand interaction isdue to the negative ligands surrounding the positive charge of themetal. This causes energy changes, and the degeneracy of the five dorbitals are then destroyed. The energy splitting is ∆o for octahedralspecies, while for tetrahedral ones it is equal to (4/9)∆o.

Complex Energy, cm-1

K3[Co(C2O4)3]∙3H2O 2.7x103

[Co(en)3]Cl3 2.2 x105

Consulting the spectrochemical series in the text, it can be observedthat oxalate is a stronger field ligand compared to ethylenediamine.

12. Discuss how changing the metal M, while keeping the same set ofligands L, affects the value of ∆o. Cite specific examples and valuesfrom your data.

Changing the metal center M while using the same set of ligandsaccounts for a drastic difference between the ∆o of the two metals. Thereason for this is the difference in charge density of the two metals.Chromium is a smaller atom compared to Cobalt. The energy of the thirdcomplex containing the metal Fe(III), however, was not calculated andtherefore cannot be used for comparison. Using this rule, Fe(III)would be in between chromium and cobalt (Cr < Fe < Co).

Complex Energy, ∆o (cm-1)K3[Co(C2O4)3]∙3H2O 2.7 x103

K3[Cr(C2O4)3]∙3H2O 2.7 x105

The energy of the Cobalt complex is only 1% of that of the Chromiumcomplex. The cobalt complex undergoes forbidden transitions thataccount for the very low intensity peak.

13. Discuss the similarities and differences of the spectra generatedfrom octahedral and tetrahedral complexes.

Tetrahedral complexes, unlike octahedral ones, do not have aninversion center. This means that they are not governed by the Laporteselection rule (d -> d transitions). This results to tetrahedralcomplexes that usually produce more intense peaks, or absorb moreenergy, compared to octahedral ones. There is also no such thing aslow-spin or high-spin Td complexes, this only happens to octahedralones.

14. Provide a spectrochemical series of the ligands based on theexperiment you performed. Compare with this the spectrochemical seriesgiven in your text.

The spectrochemical series from literatures or textbooks2 is givenbelow:

I- < Br - < S2- < SCN- < Cl- < NO3- < F- < OH- < C2O4

- < H2O < NCS- < CH3CN <NH3 < en < bipy < phen < NO2

- < PPh3 < CN- < CO

Ligands used in the experiment:

C2O42- , H2O

Ethylenediamine Ammonia NH3

The series that can be deduced from the experiment is as follows:

oxalate < en < NH3

It can be said that the ligands follow the theoretical series in thisexperiment.

15. Provide the Irving-William’s series of the metals based on theexperiment you performed. Compare this with the series found ininorganic books.

Mn(II) < Ni(II) < Co(II) < Fe(III) < Cr(III) < Co (III) < Ru(III) <Mo(III) < Rh(III) < Pd(II) < Ir(III) <

Pt (IV)

The one obtained in the experiment was Cr(III) > Co(III).Experimental errors and ligand effects could have caused this.

16. Assign the important peaks in the IR spectrum of each sample.Compare the frequency of these functional groups if these are presentin organic compounds.

Figure 1. IR Spectrum for Potassium trioxalatochromate(III) trihydrate(K3[Cr(C2O4)3] • 3H2O)

Figure 2. IR Spectrum for Potassium trioxalatoferrate(III) Trihydrate(K3[Fe(C2O4)3] • 3H2O)

Figure 3. IR Spectrum for K3[Co(C2O4)3] • 3H2O potassiumtrioxalatoferrate(III) trihydrate

Figure 4. IR Spectrum for Tris(ethylenediamine)cobalt(III) Chloride[Co(en)3]Cl3

Figure 5. Pentaamminechlorocobalt(III) Chloride [Co(NH3)5Cl]Cl2

Figure 6. IR Spectrum for Pentaamminenitritocobalt(III) Chloride[Co(NH3)5ONO]Cl2

Figure 7. IR Spectrum for Pentaamminenitrocobalt(III) Chloride[Co(NH3)5NO2]Cl2

17. Compare the minimized energy of the molecules with the same metalcenter. What can be concluded from these results?

18. Compare the minimized energy of the molecules with the sameligand. What can be concluded from these results?

Oxalato complexes that bound with Fe(III) and Co(III) compared toCr(III) have minimized energy that are very large in magnitude.

19. Relate the calculated ∆o with the minimized energy of themolecules.

The magnitude of the minimized energy depends on what strength of thefield of ligand is. The weaker the ligand, the lower the magnitude ofthe minimized energy..

20. Discuss the effect on the minimized energy and ∆o of the molecule[NiL2(PPh3)2] when L = Cl- and when L = CNS-

Cl- is a weaker field ligand compared so CNS-, so the magnitude of the∆o would be smaller with L = Cl- compared to that of when L = CNS-

22. Compare the bond lengths of Co-O (oxalato complex) and Co-N (encomplex). Explain the difference.

The bond lengths are for Co-N for the en complex and Co-O complex arealmost the same, although Co-O is slightly longer. This can be accounted for the higher electronegativity of O compared the N atom.

REFERENCES

[1] Coordination Chemistry. Origin of Color in Complex Ions.http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Coordination_Chemistry/Complex_Ion_Chemistry/Origin_of_Color_in_Complex_Ions(accessed Nov 10, 2014)[2] Depaul Education. The Spectrochemical Series.http://chemistry.depaul.edu/wwolbach/422/The%20Spectrochemical%20Series.pdf(accessed Nov 10, 2014)