Equilibria of mononuclear oxomolybdenum(VI) complexes of triethanolamine

Equilibria of mononuclear oxomolybdenum(VI) complexes oftriethanolamine.

A multinuclear dynamic magnetic resonance study of structureand exchange mechanisms

Gabor Szalontai a,*, Gabor Kiss b, Laszlo Bartha b

a NMR laboratory, University of Veszprem, H-8200 Veszprem, Pf. 158, Hungaryb Department of Hydrocarbon and Coal Processing, University of Veszprem, H-8200 Veszprem, Pf. 158, Hungary

Received 4 November 2002; received in revised form 10 December 2002; accepted 10 December 2002

Abstract

1D and 2D 1H and 13C NMR spectra of the assumed [MoO4(TEA)]2� complex recorded in DMSO at variable

temperatures clearly indicate one free and two bound hydroxyethyl arms. The free arm of the ligand readily exchanges

with the two metal-bound arms. Under such conditions the triethanolamine (TEA) acts as a bidentate ligand. The

presence of water accelerates the exchange, which at higher water content involves the free ligand too. In organic

solvents the binding strength of the hydroxo groups to the molybdenum is weaker than that of the water molecules. A

plausible structure is confirmed by 14N, 17O and 95Mo measurements and an exchange mechanism based on the

existence of an eight-membered relatively rigid chelate ring is suggested.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Molybdenum(VI); Triethanolamine complexes; Multinuclear magnetic resonance spectroscopy; Structure; Exchange

mechanism

1. Introduction

Molybdenum is known to form stable chelates

with many tripod ligands such as various amino-

polycarboxylates [1], triamidoamines [2], tris(2-

ethylamino)amines [3], or with ligands which

contain facially arranged amine, carboxylate,

and/or heterocyclic nitrogen functional groups [4]

in mono-, di- or polynuclear complexes. Trietha-

nolamine (TEA) also has attracted interest in

metal coordination chemistry [5]. However, to

the best of our knowledge, no study on molybde-

num(VI) complexes with TEA has been reported

so far. This may be related to the general

observation that hydroxyl coordination is less

favoured than carboxylate coordination, at least

in aqueous solutions. Ligands with three identical

* Corresponding author. Tel.: �/36-88-422-022/4356; fax: �/

36-88-421-869.

E-mail address: [email protected] (G.

Szalontai).

Spectrochimica Acta Part A 59 (2003) 1995�/2007

www.elsevier.com/locate/saa

1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S1386-1425(02)00445-6

mailto:[email protected]

arms, e.g. TEA or nitrilotriacetate (NTA) have

been mostly used to prepare mononuclear metal

complexes by forming up to three stable five-

membered chelate rings. In solution the stability of

such chelate rings depends on several parameters

such as pH, solvent, temperature or the presence

of other competitive binding groups. While at

pH]/6�/8 formation of mononuclear species is

favoured (e.g. tricyclic metalatrane [5]) at lower

pH values relatively stable di and polynuclear

complexes of NTA have also been reported [6]. It

is also known that while the mononuclear molyb-

date anion in alkalic solution is tetrahedral,

oxomolybdate(VI) complexes with chelating li-

gands normally have octahedral or distorted

octahedral configuration [7].

We found that in the presence of TEA MoO3

dissolves in water (in fact it transforms to tetra-

hedral MoO42� ions [8]) and forms a solid crystal-

line complex of unknown structure. The pH of the

solution is basic (pH �/8�/10) (Scheme 1).In an early work on a similar system ([MoO3�/

NTA]3� dissolved in methanol), based on the

observation of two bound and one free arms and

slow exchange among them, Miller and Went-

worth [1] suggested two possible exchange me-

chanisms, one with Mo�/N bond rupture

(favoured) and an another one, without it. Later

on to be able to explain the single 17O resonance

observed in the 17O NMR spectrum of [MoO3�/

NTA]3� and [MoO3�/iminodiacetate]2� for the

Mo�/O oxygens scrambling of these oxo ligands

was assumed [4]. The proposed mechanism [4] was

based on nucleophilic attack of the free glycinate

arm and retention of one five-membered chelatering during the interchange of the oxo positions.

Our aim was to conduct a detailed multinuclear

magnetic resonance study in order to investigate

the coordination mode of the hydroxyethyl arms,

to determine the solution state structure of the

crystalline complex formed, the most likely me-

chanism of its formation and details of the

equilibria involved. For this purpose we haveused both dipolar and quadrupolar nuclei.

2. Experimental

2.1. Preparation of the complex

It was found that the MoO3 forms water-soluble

complexes in the presence of water and different

type of amino-alcohols (mono-, di-, TEA). In case

of the TEA we could produce solid crystals by

evaporation of the water. With 1:3:18, Mo:TEA:-

water molar ratio we got yellowish solution after

heating for 4 h at 100 8C. After the evaporation of

the water and removing the unreacted TEA bywashing the solid phase with isopropylalcohol we

got off-white crystals.

Elemental analysis of the crystals gave a com-

position of C6H17N1O7Mo1 what corresponds to

one ligating TEA to a MoO42� unit. (Calculated:

C, 21.1; H, 5.0; N, 4.1; O, 41.6; Mo, 28.1.

Measured: C, 22.3; H, 5.1; N, 4.3; O, 34.6; Mo,

32.1%.) Powder X-ray diffraction spectra weremade on a Philips PW10/20 instrument under

Scheme 1.

G. Szalontai et al. / Spectrochimica Acta Part A 59 (2003) 1995�/20071996

CuKa irradiation and proved that the structure ofthe solid complex was uniform.

2.2. Spectroscopy

The NMR spectra shown were recorded on a

Varian UNITY 300 spectrometer operating at 7.04

T. For 1H (300 MHz), 13C (75.43 MHz), 17O

(40.67 MHz) spectra we used a 5 mm VarianBroad Band probe and standard VARIAN software

(VNMRS 6.1C). In the case of 14N (21.67 MHz) and95Mo (19.54 MHz) a 10 mm low-frequency Varian

probe was used. The residual protons or carbon-13

atoms of the deuterated solvents were used as

internal references in case of proton and carbon-13

spectra (1H: DMSO�/2.5 ppm and 13C: 39.5 ppm

relative to TMS). Unless noted otherwise spectrawere recorded at 293 K. Oxygen absorbed in the

solvent has not been removed from the sample.

Chemical shifts of 17O, 95Mo and 14N were

referenced to internal H2O and, by the substitution

method, to MoO42�, and CH3NO2, respectively.

2.3. Concentrations

It was possible to solve about 50 mg of the

complex in DMSO, even more than this in D2O,

but practically nothing in any other organic

solvents. A solution containing 10�/50 mg of the

complex in 0.6 ml solvent was stable for months.

The deuterated DMSO we have used thorough the

study was purchased from Aldrich with water

content of about 0.01�/0.05%.The IR spectra were recorded on a Biorad IR

spectrophotometer.

3. Results and discussion

3.1. 1D and 2D 1H NMR studies

Spectra of the assumed reaction product[MoO4(TEA)]2� recorded in D2O indicated a

symmetric species with a 3-fold axis of rotation

(C3v) (see Fig. 1b). By comparison with TEA*HCl

we could prove unambiguously (see Fig. 1c and b)

that this symmetric species is the protonated form

of the free ligand (i.e. TEAH� MoO42�).

In DMSO (the solution is slightly acidic, pH 6.6)

however, the symmetry is lost, we observe two

bound and one free arm for the TEA (Fig. 1a).

Chemical shift data obtained in DMSO and

D2O solutions are collected in the Table 1. The

chemical shift non-equivalence of the geminal

protons of both methylene groups of the bound

arms suggests the existence of a rigid asymmetrical

structure. Five-membered chelate rings come first

in mind. This, however, would involve the co-

ordination of both the amine nitrogen and two of

the three alcoholic OH groups.

Larger chelate rings can also be envisaged but

some rigidity of the ring must be maintained.

Exactly the same observations have been reported

for the solution state behaviour of the correspond-

ing NTA complex [1], in addition in that case the

obtained single crystal structure proved the ex-

istence of five-membered chelates in the solid state.

At the same time methylene protons of the

unbound arm should rotate freely and are there-

fore chemically and magnetically equivalent at this

frequency (A2X2 spin system). In DMSO they are

shifted to low frequencies by about 0.9 (OCH2)

and 0.4 (NCH2) ppm, relative to their bound

counterparts. At about 3.34 ppm we observe two

overlapping singlets, one of them should be

assigned to the water content of the solvent,

whereas the other one, most probably, to the

alcoholic OH groups. By increasing the sample

temperature the signals are getting broader, the

fine structure assigned to the bound arms collapses

first (at about 313 K) whereas that of the free arm

follows only at about 353 K. A possible inter-

pretation can be the assumption of two exchange

processes, the lower energy process can be a ring

inversion, and the higher energy one can be

associated with bond rupture.

As proved by various 1D and 2D exchange

experiments (see Fig. 2b) the bound and unbound

arms are in exchange slow on the NMR time scale.

Furthermore, an exchange exists between protons

of the geminal proton pairs too, what indicates

either a simultaneous ring inversion or any other

rearrangement that causes the exchange of the

chemical environment of the assumed axial and

equatorial protons.

G. Szalontai et al. / Spectrochimica Acta Part A 59 (2003) 1995�/2007 1997

There is a one-proton broad signal at 7.6 ppm

which produces exchange cross peaks with the

water signal at 3.3 ppm and gives significant NOE

enhancements to the protons of the �/NCH2�/

groups of both the bound and unbound arms.

All this, together with the lack of NOE enhance-

ments to the �/OCH2�/ protons, suggest NH or

more likely NH� proton, however, if so, the

Fig. 1. 1H NMR spectra of the complex [MoO4(TEA)]2� recorded at room temperature in DMSO-d6 (a), in D2O (b) and spectra of

the free TEA recorded in D2O (d) and its HCl salt (c) also recorded in D2O.


nitrogen is not bound to the metal, what definitely

contradicts with the existence of five-membered

chelate rings!Since even after a 10-fold dilution of the sample

we got the same results the exchange must be of

intramolecular nature. Unambiguous assignments

of the geminal protons to the bound arms (see the

Table 1) were possible by standard COSY [9] and

EXSY [10] experiments (see Fig. 2a and b).

However, assignments of the ring protons to the

assumed quasi-equatorial and quasi-axial posi-tions are tentative only.

3.2. 13C NMR studies

In pure water, like the 1H spectrum, only the

protonated TEA can be observed (see the two

overlapped lines at 52.8 and 52.7 ppm in Fig. 3a).

However, at room temperature in DMSO (not

dried previously) the spectrum clearly shows two

bound (73.2 ppm �/OCH2 and 60.2 ppm �/NCH2)and one unbound (57.7 �/OCH2 and 56.5 �/NCH2

ppm) arms for the ligand (see Fig. 3b). Values for

the free ligand in DMSO (�/CH2O�/ 59.2 ppm and

CH2N 56.0 ppm) have been reported recently [11]).

The 13C chemical shift values and their assign-

ments, including the data of the HCl salt too, are

collected in the Table 1.

By adding water to the crystals dissolvedoriginally in DMSO the spectrum changes gradu-

ally (see Fig. 3b�/d), line widths are increasing and

the free ethanolamine appears already at a water

content of about 5%. Exchange spectra recorded at

this water content and above prove that the

exchange is not restricted to intramolecular pro-

cesses, but it involves the free ligand too. At water

content higher than about 50% only the proto-

nated ligand can be detected. Adding to theDMSO solution some HCl resulted in the forma-

tion of the HN�R3 salt of the ligand.

As expected, 13C,13C EXSY spectra (see Fig. 4)

revealed that the exchange is slow on the 13C

NMR time scale too. The bound�/free exchange

rate, kbound, free calculated from a series of EXSY

experiments [10] using short mixing times and

linear approximation is about 40 per s (Fig. 4).By adding free TEA to the DMSO solution of

the complex we could prove that (i) the unbound

arm cannot be assigned to the free TEA (ii) the

presence of free TEA, however, causes line broad-

ening of the bound and free arms while itself is

practically not affected (iii) the exchange rate is

increasing by increasing the amount of the added

TEA or by elevating the temperature. The pre-sence of excess TEA is expected to shift the

protonation equilibrium.

3.3. 95Mo NMR studies

This quadrupolar nucleus (I�/5/2, quadrupole

moment 0.12�/10�28 m2) is almost routinely used

nowadays to characterise the oxidation state of the

metal and the coordination sphere around it [12�/

16]. Line widths being entirely controlled by thequadrupolar relaxation mechanism reflect the

symmetry relations around the Mo atom [13].

The free MoO42� ion has tetrahedral symmetry

(Td), on coordination of a monodentate ligand this

will be lowered to C3v and further to C2v on

coordination of a bidentate ligand [17]. It is

Table 11H and 13C NMR chemical shift data of the complex, the free ligand (TEA) and its HCl salt (in ppm relative to external TMS)

Free arm Bound arm TEA (free basis) TEA*HCl

�/O�/CH2�/ �/N�/CH2�/ �/O�/CHAHB�/ �/N�/CHAHB�/ �/O�/CH2�/ �/N�/CH2�/ �/O�/CH2�/ �/N�/CH2�/

1H NMR* 3.6 2.96 4.6 and 4.46 3.46 and 3.16 3.58a 2.66a 3.83a 3.36a

13C NMR* 57.7 56.5 73.2 60.2 59.2a 56.0b 57.2c 56.3c

* DMSO-d6, 293 K, 300 and 75.4 MHz, �/10�/30 mg/ml.a Recorded in D2O.b Recorded in DMSO.c Recorded in DMSO-d6�/10% H2O.


generally agreed on that tetrahedral molybdenum,

in accord with the higher site symmetry, exhibit

narrower lines relative to the octahedral ones [18],

there are, however, noticeable exceptions [14].

Fig. 2. 1H, 1H absolute value COSY 908 (a) and NOESY (b) spectra of the [MoO4(TEA)]2� complex recorded in DMSO-d6 at 300

MHz, 293 K. NOESY experiment: the mixing time was varied between 0.2 and 0.8 s, both the exchange and the antiphased NOE cross

peaks are shown in black, but they are indicated by separate arrows. COSY 908 experiment: absolute value spectrum, weighting

function, sine bell; data points, 1k*1k.


In DMSO at 293 K (pH 6.3) a singlet (Dn1/2�/

160 Hz) is observed for the [MoO4(TEA)]2�

complex at 129.7 ppm relative to the MoO42�

anion (Fig. 5a). This line width suggests relatively

large value for the asymmetry parameter of the

electric field gradient tensor around the 95Mo

nucleus. When dissolved in D2O the pH is some-

what higher (6.6) and two signals, a broad one at

36 ppm (Dn1/2�/614 Hz) and a much sharper one

(Dn1/2�/50 Hz) at �/2.1 ppm are observed at

Fig. 3. 13C{1H} NMR spectra of the [MoO4(TEA)]2� complex recorded at 75.4 MHz (293 K) in pure D2O (a), in DMSO-d6 (b), in

DMSO-d6�/5% water (c) and in DMSO-d6�/10% water (d). Integral ratio of the bound and free arms correspond to 2:1 (see (b) and

(c).


ambient temperature (see Fig. 5b). At elevated

temperature, however, the broad and the sharp

signals tend to coalesce (Fig. 5a�/c).Exactly the same observations have been re-

ported earlier [19] when acidifying an alkaline

solution of molybdate.

We interpret these spectra as indications for

octahedral symmetry of the complex in DMSO

whereas in D2O at pH 6 or below it transforms

into an octahedral and a most probable tetrahe-

dral species which are in exchange with each other.

The broader signal is presumably [Mo7O24]2� [20],

see Fig. 5c for the spectrum of (NH4)6[Mo7O24]

recorded under identical conditions. However, one

has to keep in mind that under such conditions the

formation of mono or diprotonated species of

MoO42� (it is known that even the H2MoO4 is

octahedral [7]) can also happen.

3.4. 14N NMR studies

This moderately quadrupolar nucleus (I�/1,

quadrupole moment 1.6�/10�2�/10�28 m2) was

expected to show changes of chemical shift and

line width upon coordination of the nitrogen to the

molybdenum [21]. While in DMSO the free ligand

and its HCl salt exhibit broad signals (Dn1/2�/

38009/50 Hz) at �/340 and �/3509/6 ppm (relative

to CH3NO2), respectively, the complex shows

somewhat narrower signal (Dn1/2�/27009/50 Hz)

at �/3239/5 ppm. In our view the observed 25�/30

ppm high-frequency shift of the signal is not a

convincing evidence for the nitrogen coordination.

Judged on its magnitude the origin can be at least

partly, a steric effect too or can be caused by

changes of the C�/N�/C bond angles upon the

chelate formation.

Fig. 4. 13C{1H}, 13C{1H} exchange spectrum (EXSY) of the MoO4(TEA)]2� complex recorded at 75.4 MHz (293 K) in DMSO-d6.

Mixing time 0.4 s.


3.5. 17O NMR studies

Fortunately this nucleus (I�/5/2) is only weakly

quadrupolar, its quadrupole moment is �/2.6�/

10�2�/10�28 m2. To be able to understand the

fine details of structure and exchange one need

information about the innermost coordination

sphere around the molybdenum, i.e. about the

oxygen atoms. To follow the fate of the oxygen

atoms of the MoO42� anion we labelled it by

dissolving in H217O (20%), this way we have

achieved about 5�/10% labelling (the exchange

readily takes place within 30 min). Liquid phase17O NMR spectra of the 17O labelled

[Mo17O4(TEA)]2� are shown in Fig. 6.

In DMSO at ambient temperature two relatively

sharp signals (B/130 Hz, T1�/T2B/0.0003 s) are

observed at 876.4 and 841.4 ppm relative to H2O,

these chemical shifts are very characteristic of the

terminal oxygen bound to molybdenum [22,23].

Fig. 5. 95Mo{1H} spectrum of the [MoO4(TEA)]2� complex (a) recorded at 19.544 MHz (293 K) in DMSO-d6, pH 6.6 (b) recorded in

D2O (293 K). (c) spectrum of the heptamolybdate [NH4)2Mo7O24] recorded in D2O (293 K), pH 5�/6; 10 mm sample tubes were used,

chemical shift is given in ppm relative to MoO42� ion.


Ranking of trans-influencing ability for common

ligand groups has been reported earlier [4]. The

sequence follows the expected electron-donating

power of the ligands, while the RO� group has

medium strength H2O and ROH groups are the

weakest in the ability order [4]. It is noteworthy

that in cases of trans-coordinating carboxylato

ligands the 17O chemical shift range is somewhere

between 680 and 710 ppm [4], in case of an ROH

group, in accordance with its weak trans-influen-

cing ability, the observed shifts are between 800

and 900 ppm. Making use of the nearly linear

correlation between the Mo�/O bond lengths and17O chemical shifts proposed by Miller and Went-

worth [24] bond lengths of about 1.79/0.03 A can

be estimated for the terminal Mo�/O bonds from

the observed chemical shift values.

The signals observed do not show measurable

exchange with each other, at least during the short

time what the fast T2 relaxation permits. Note that

the 17O enriched MoO42� anion would exhibit a

sharp resonance (Dn1/2 �/6 Hz) at 533 ppm relative

to H2O but this signal is not present in DMSO.

Since no signal is observed in the expected range of

the bridging oxygens (between 200 and 500 ppm

[25]) dimeric nature of the complex or the forma-

tion of oxo-bridged Mo2O52� units [4] can be

excluded. (It is reasonable to assume that a

bridging oxygen, if exists, must come from the17O labelled MoO4

2� anion). Some labelled oxygen

disappears from the coordination sphere of the

molybdenum by OH� exchange and gives a small

H2O signal near 0 ppm. Gradual addition of water

to the DMSO solution (Fig. 6b) resulted in

substantial broadening of the resonances at water

content higher than 25% or at elevated tempera-

tures the signals practically disappear.

The temperature dependence of the 17O spectra

was studied on a 17O labelled sample in the

temperature range of 20�/120 8C. At room tem-

perature line widths of the two signals at 876 and

842 ppm are 105 and 127 Hz, respectively. By

Fig. 6. 17O{1H} NMR spectra of the [MoO4(TEA)]2� complex recorded (a) at 40.670 MHz (293 K) in DMSO-d6, pH 6.6, 5 mm

sample tube (b) same as (a) except that 15 v/v% water was added to the DMSO-d6 solution. Chemical shifts are given relative to H2O.17O labelled MoO4

2� (5�/10%) was used in the reaction.


increasing the temperature first they show somedecrease (they are 84 and 115 Hz at 40 8C),

however, at 60 8C and above we observed sub-

stantial increase (at 120 8C the relevant values

were already 305 and 588 Hz). While the decrease

can be explained by the expected lengthening of

the 17O relaxation times at higher temperature, this

effect is outbalanced by the exchange at 60 8C and

above. The full coalescence is somewhere between160 and 180 8C.

3.6. Infrared spectroscopy

The cis-dioxo arrangement of the oxygens was

also confirmed by infrared measurements, the

spectra recorded in DMSO exhibit two strong

n (Mo�/O) bands at 925 and 901 cm�1 which are

characteristic of the cis-[MoO2]2� fragmentsasymmetric and symmetric vibrations, respec-

tively, [26].

4. Conclusion

4.1. Time averaged structure in DMSO at room

temperature

The results point to a monomeric octahedral

oxomolybdenum(VI) complex, in which two of thethree arms of the ligand form an eight-membered

chelate ring (two coordinating alcoholic OH), one

hydroxyethyl arm is free all the time. Two non-identical perhaps covalently bonded terminal �/O

atoms occupy two additional positions, whereas

O� groups are bound to the remaining two sites.

Based on their different chemical shifts one of the

terminal oxygen is thought to be in apical the other

in equatorial position. The latter is cis and trans

related to the exchanging two hydroxyethyl arms.

As a sine qua non inversion of the chelate ringmust be slow. The relative rigidity of the ring is

thought to be due to the protonation of the

nitrogen atom, a feature common to six-membered

rings, but less known for eight-membered ones. As

indicated by variable temperature 1H measure-

ment the ring inversion itself is a low-energy

process (Scheme 2).

4.2. Possible exchange mechanism

Earlier consecutive dissociation of the metal�/

hydroxo and metal�/nitrogen bonds followed bynitrogen inversion were considered as precondi-

tions for the C3 or higher symmetries observed for

aminopolycarboxylate complexes of Zn(II) Cd(II)

and Pb(II) [27,28]. No doubt the stability of the

possible Mo�/N bond is of crucial importance. The

mechanism proposed by Miller and Wentworth for

[MoO3NTA]3� is based on temporary dissociation

of the Mo�/N bond and possible water involve-ment but the inertness of the two Mo�/carboxylate

bonds are maintained. Just opposite in a trigonal�/

Scheme 2.


twist mechanism or in the one proposed by Reilleyet al. [4] the Mo�/N bond is fixed through the

cycle. Since the labilities of the Mo�/carboxylate

and the assumed Mo�/hydroxo bonds can be

different, the analogy, if any, should be taken

with caution. Relatively stable Mo�/N bonds have

been observed quite recently in dioxomolybde-

num(VI) complexes with bidentate nitrogen donor

ligands [29].To give a reasonable explanation for the two

simultaneous exchange processes observed besides

the exchange of the free and bound arms we have

to assume a simultaneous nitrogen inversion too

(see Scheme 2). The driving force behind the

nitrogen inversion can be the exchange of the

bound protons with the free ones

([MoO42�R3NH�]U/[MoO4

2�R3N]�/H�). (Pleasenote that in DMSO there is always some water and

remember the one-proton NH� signal observed in

the 1H spectrum in DMSO at 7.6 ppm). This is in

line with the observation that the presence of free

water accelerates the exchange. The OH� groups

or the water molecules are seemingly stronger

coordinating partner than the alcoholic OH

groups, therefore, in the presence of small amountof water one coordinated hydroxyethyl is replaced

temporarily by an OH� group which is soon

replaced by an another hydroxyethyl OH (see

Scheme 2). Due to the nitrogen inversion, what

may rotate the single bound TEA by about 1208the newly bound arm is often, if not always, the

one, which was hitherto free. Although the one-

way nature of the rotation is not proved, one canenvisage a stepwise rotation of the ethyloxy arms

too. Larger excess of water leads to complete

dissociation of the TEA from the MoO42� ion as

confirmed by 1H, 13C, 95Mo and 17O NMR

spectra. The similarity of the observed exchange

phenomena between the [MoO3NTA]3� and

[MoO4(TEA)]2� complexes raises questions about

the mechanism proposed by Miller and Went-worth [1], (e.g. no evidence was presented for the

existence of Mo�/N bond in liquid phase, the

observation of the NH proton may have been

hampered by the solvent used). Nevertheless with-

out further detailed comparative study carried out

in identical solvent it is too early to say more than

that.

Acknowledgements

The authors gratefully acknowledge the finan-

cial help of the Hungarian Scientific Research

Fund (OTKA, grant number T 34355), thank

Professor Janos Mink (VE, Veszprem) for valu-

able remarks on the IR spectra and Dr Fritz E.

Kuhn (TU, Muchen) for critical comments and

suggestions on the manuscript. The kind helps ofLaszlo Hajba in recording the IR spectra, Dr

Zsuzsa Hartyanyi in recording the powder diffrac-

tion spectra and Balint Szalontai (NMR sample

preparation) are also acknowledged.

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Equilibria of mononuclear oxomolybdenum(VI) complexes of triethanolamine

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Transcript of Equilibria of mononuclear oxomolybdenum(VI) complexes of triethanolamine