Isotopic 18O exchange between solvent water and aquo molybdenum ions

Poiykhn Vol. 5, No. I/Z, pp. 487-495, 1986 Printed in Great Britain

ISOTOPIC

0277-5X37/86 $3.00+ .@I Pcrgamcln Press Ltd

180 EXCHANGE BETWEEN SOLVENT AQUO MOLYBDENUM IONS

GARRY D. HINCH,

DONALD

E. WYCOFF

WATER AND

and R. KENT MURMANN*

Department of Chemistry, University of Missouri, Columbia, MO 65203, U.S.A. (Received 17 June 1985) Abstract-Recent applications of I80 transfer and kinetic exchange experiments coupled with X-ray crystal structures has led to the establishment or confirmation of aqueous structures of some aqueous molybdenum ions. The mechanisms by which molybdenumcoordinated OH,, OH- and 02- are exchanged with solvent are not well-understood. Now there are several ions which have been carefully studied and some suggestions as to mechanism can be made. A summary of oxygen transfer and kinetic exchange studies is made for the aquo ions MOO;-, Mo,O, 2+ , Mo304(0H2)z+ and several of their complexes. Interpretation of oxygen transfer accompanying reduction to lower oxidation states gives some structural and kinetic information about the reduced ion. Some new data are presented on complexes of these oxidation states which allow an estimate of the rate behavior of 0x0 and bridged-hydroxo groups in the neutral and basic region not accessible with the uncomplexed aquo ion.

Studies on the isotopic ‘so exchange between solvent water and aquo, hydroxo or 0x0 groups coordinated to metal ions can give information about the number of oxygens of a particular type surrounding the metal ion, the number of different types of bound oxygens, the kinetic properties of these oxygens and the mechanism by which exchange occurs. A knowledge of the oxygen exchange reaction kinetics for an aquo ion : G-I,O*)

+

CM@HJnI”+ +

1

n(H,O) + [M(*OH,),]“+

is necessary for an understanding of substitution reactions of the ion. Likewise, for an 0x0 ion or hydroxy-bridged species, the rate of isotopic exchange may be closely related to the rates of substitution reactions on it. Isotopic IsO methods utilizingmass spectrometry are very precise and accurate even by today’s standards but are not suitable for rapidexchanges. In a few cases reactions with t,,,s of less than 2 s have been kinetically studied but under usual experimental conditions half-times of greater than 1 min are needed for accurate results. Thus other methods

* Author to whom correspondence should be addressed.

such as NMR, IR, sound absorption, laser Raman and mass spectra of the parent molecule or fragment are used when applicable for the more rapid exchanges. The forerunners in this field were Cohn and Urey (1938)l and Hall and Alexander (1940).2 The area was significantly advanced by the innovative studies of Taube et al. (1950-)3 and since that time a slow stream of results has been published by a few small groups throughout the world. A review4 appeared in 1982 covering all metals and some generalizations were suggested for predicting rate laws along with mechanistic interpretations. Since that time, new results have been obtained on molybdenum aquo species and complexes and offer an insight into the structural and kinetic variety obtainable with this metal. Thus we will describe the results of the application of ‘so tracer and exchange techniques to the aqueous ions of molybdenum. EXPERIMENTAL In most cases, two types of measurements are made : (a) determination of the number of oxygens coordinated to a metal which are slow to exchange, and (b) measurement of the rate of exchange of those oxyben atoms. For both types the basic experiment is the same. A solution of the ion to be studied at

487

488

G. D. HINCH et al.

equilibrium is treated with a small quantity of highly tetrahedral ion which appears to be monomeric in enriched “0 water to change the overall “0 the moderately alkaline region.**’ The latter study,g content of the solvent. At times very close to that of which included WOi- was restricted to the alkaline mixing(experiment a) or at later times (experiment b) region so as not to involve protonated species or either the solvent or the complex ion is sampled. oligomers derived from them. Von Felton et aLg Generally the complex ion is sampled and its total precipitated the ion with Ba’+ as anhydrous ‘*O content measured. For this purpose it is BaMoO, and was essentially necessary to be able to quickly remove the ion from zero, suggesting that the only important ion in solution, as a well-defined solid, at low temperature solution has four oxygens surrounding it and that the without losing or catalyzing the exchange of the precipitate has all of the characteristics, which were oxygens under investigation. The precipitate should given earlier, necessary for a quantitative study. The be non-hydrated, form very quickly and once formed isotopic measurement was carried out on highly not exchange its oxygen with the solvent during the purified CO obtained by inductively heating the salt period of collection, purification and drying. Its with graphite in a vacuum system. exact composition has to be known either from The half-time for exchange was short, in the region analysis or better from a single-crystal X-ray of 5-20 s at 5°C and followed the rate equation : determination. R/[MoO:-] = k,+k,[OH-] For isotopic analysis, the oxygen in the solid salt has to be converted to a pure simple gas. CO2 is rather well over the range generally used and the conversion to it done in a non[OH-] = 3-150 x 1O-3 M. quantitative manner by heating the sample with Hg(CN), and HgCl, in a sealed glass tube for an hour The activation parameters were AH* = 62.8(2.5) at 425°C.’ During this time all oxygens become and 70.3(5.0) kJ M-’ and AS* = -43.5(9) and isotopically equivalent and the CO* is separated -2.5(13) J K-’ M-’ for k, and kr, respectively. from the other products by preparative gas The k. term for MOO:- and WOi- are nearly the chromatography. After further purification in a same, about 0.4 s-r in the region of 25°C but much vacuum line the 46/(44 + 45) ratio is measured with a larger than that for CrOa-, which is about 3 x lo-’ mass spectrometer. Generally the enrichments are s-l. The main difference is attributed to the larger low fless than 3 x normal (0.6x)] and ratio values for the enthalpy of activation with CrOz-, measurements can be made to 0.1% of enriched reflecting the “tighter” Cr-0 bonds compared to minus normal. Usually it is necessary to know the those of Mo(V1) or W(V1). The similarity of the enrichment of the solvent which is accomplished by molybdenum and tungsten rates is fortuitive due to the equilibration of a portion ofit (3 days, 25°C) with an unusually large negative AS* for tungsten which compensates for its small value for AH*. The kinetic a small amount of CO1 in a sealed tube, separating and measuring the ‘*O content of the purified COZ. solvent H/D isotope effect for k,, is 3.6 and for kl is From the amounts of water and CO, and the 2.3, about the same as for the tungsten analogue, suggesting proton transfer in the rate-determining isotopic fractionation factor the solvent enrichment steps. An isokinetic graph, AH* vs AS, for k, for is calculated.‘j ions, WOZ-, MOO:and The rate of exchange for a single exchange is first- three tetrahedral is linear, suggesting a common rateorder (McKay plot),’ i.e. In (1 - F) vs time is linear, ReO;l,” determining step. It is suggested that the mechanism slope = kobsand : for the k, term is of the solvent-assisted dissociative

=

k[]"[]"...,

where a and b are the concentrations of exchanging oxygen atoms and solvent oxygen atoms, respectively. Nonlinear behavior of McKay plots is indicative of a chemical reaction occurring or more than one type of oxygen in the ion under study. RESULTS

AND

DISCUSSION

Mo(V1) The first aqueous molybdenum ion to be kinetically studied by I80 techniques was MOO:-, a

type, while for the k, term the metal ion expands its coordination sphere as the yl-oxygen leaves. Since it has not been possible to determine the order with respect to water, a more precise suggestion as to the mechanism cannot be made. A wide and important region of study for this ion is left open, that of the neutral and faintly acid areas. Here HMoO; would be expected to dominate the kinetic expression and the rate should be accelerated dramatically as we

Isotopic 180 exchange between solvent water and aquo molybdenum ions

489

approach acidic conditions similar to that seen for other oxy-anions. Interpretation of “0 results would be highly dependent on knowing the exact species and their concentration as a function of pH and total [MO].

MO(V) The next study described is for the ion MoOClg-, at the other extreme ofthe acid-base scale.’ ’ The ion exists only at high [acid] and [Cl-] according to spectral studies. Structural information about the ion in solution comes from analysis, X-ray crystal structures and spectral evidence, mostly in the solid state. The ‘so exchange studies could be conducted without interference from other forms of Ma(V) in 9.2 M HCl down to 2.6 M HCl+8.9 M LiCl. Cs,MoOCl, was used as the precipitating agent and CO, as the analyzed gas. The zero-time exchange showed about 5% induced exchange which could not be reduced by experimental variations. Since the structure of the anhydrous precipitated salt is known from X-ray diffraction and analysis and essentially no exchange takes place after short times of contact with enriched solvent, the ion cannot expand its coordination number with solvent water in solution. For instance [CI,MO(OH)~]~cannot form reversibly. The experiment also proved that there is l.O+O.l slowly exchanging oxygen in the aqueous ion. The.rate of isotopic oxygen exchange is slow, t1,2 = l-2 h, and is dependent on [acid], [Cl-] and [MoOCl:-1. At constant ionic strength (LiCI) (12.1 M), kobswas nearly constant from 2 to 10 M HCl but increased significantly between 10 and 13 M. Increased [Cl-] decreased kobs if the acidity was below 10 M but not if above. A plot of kobs vs [MoOCl:-] at constant [HCl] (12.1 M) showed a linear relationship with the intercept zero at [MoOCl:-] = 0. The best fit to all of the data was given by the expression : R = kl [MoOCl: -]/[Cl-]

+ k,[MoOC1:-]2[HCl]. In these very concentrated solutions one cannot confidently conclude mechanistic paths from the form of the rate equation. However the first term, kl, is consistent with an aquation equilibrium step followed by the rate-determining isotopic exchange. The intermediate aquo ion has been isolated and its solid structure determined (Fig. 1).14 The ratedetermining step would be of the internal electronic rearrangement (IER) type which requires a labile water or OH- group cis or trans to the y&oxygen. The second term, k,, requires two complex ions and a

Fig. 1. ORTEP drawing of [MoOC14(OH,)]-.

positive acid effect in the activated state. A reasonable explanation of this data lies in first protonating a complex ion forming a hydroxo complex and then a slowly reversible reaction would produce a bridged 0x0 complex and cause isotopic exchange : [CI,MOO]~ - + Ha+4. s [CI’MoOH] -, H+ + [Cl,MoOH]-

+ [Cl,Mo012-

2 [CI,Mo-0-MoCI,]‘-

+ H,O.

There is ample evidence for the protonation of dioxo complexes in [Re(en),O,] +, [Mo(CN),0214-, and more recently even with V02+(aq).i2 Dimerization in hydrolyzed MO(V) solutions has been postulated even in strongly acid solutionsr3,14 (HCI) and it dominates the chemistry in dilute acid or neutral solution.” [MoOBr,12follows essentially the same behavior but is slightly faster. This more rapid rate probably reflects the greater tendency toward hydrolysis. This suggests that stronger ligands, i.e. SCN- or chelating ligands (EDTA4-) would lower the rate of yl-oxygen exchange. Over a wide range of mildly acidic solutions, MO(V) exists almost exclusively as a dimer, aquated Mo,O:+. The evidence behind this formulation is its diamagnetism, ion exchange properties, spectral properties and X-ray solid structures of complexes. The present evidence supports a dimeric cisstructure with three distinguishable oxygen types.

P P2’ /‘MdO’M/ I \o/ I \ Measurement of the rate of oxygen exchange with this ion was complicated by the fact that no precipitating agents were known with reasonably suitable properties. It was foundI that during the conversion to the EDTA4- complex (whose X-ray

490

G.

D. HINCH et al.

structure is known), no isotopically different solvent oxygen was introduced and the resulting ion could be quantitatively and quickly removed from solution with [Pt(en)J 4+. Thus one could “follow” the yl and bridging oxygens, but not the coordinated water since it was replaced by EDTA4-. Also, once formed, all oxygens of [Mo,O,(EDTA)]‘-, including those of the organic ligand, are very slow to exchange even in solution, For this conversion the time-zero induced exchange average was 2.9( 1.5)%.16 This established that the 4.qO.l) oxygens attached to MO in [Mo~O,(EDTA)]~came from the firstcoordination sphere of Mo(V)(aq) and not from the solvent. The two types of oxygen proved to have quite different exchange rates over a wide range of conditions. At 0°C in 0.5 M H+, 2.04(6) oxygens exchange rapidly with a half-time of about 4 m. The slowly exchanging oxygens were also counted and found to number 1.98(3) and the exchange half-time at 40°C was about 70 h in 0.5 M acid. The McKay plots were highly linear for the fast oxygens but the rate appeared to increase with time for the slow oxygen exchange. The non-linear behavior could easily be removed by having Hg(1) present which reduces Mo(V1). It was thought that either disproportionation or oxidation with traces of oxygen generated MO(W), which catalyzed the exchange. The slow oxygens were shown to increase their exchange rates with increasing [MOO:-] in the l-3 mM range and so the exchange is very sensitive to it. The fast oxygens were shown to be the yl-oxygens by laser-Raman IR spectroscopy where the spectrum was taken in 90% “0 water before and after the fast oxygens exchanged. Assignment of the 950~cm-’ region to the yl-oxygen and the 760-cm-’ region to the bridging oxygen was done on the basis of the spectra of solid complexes. Only the yl-oxygen, 950 cm- ‘, changed in a short time. The yl-oxygen exchange showed an invariance to [MozOi+] and [acid] in the ranges 0.002-0.039 and 0.01-3.1 M, respectively, and increasing [Cl-] lowered the rate constant by only a small amount. The latter effect could not be separated from that due to ionic strength changes. On the other hand the bridging oxygens followed the rate law : R/[Mo,O:+]

= k[H+]‘,

with the rate decreasing as the ionic strength or [Cl-] increased. The yl-oxygen exchange was thought to occur through a solvent-assisted dissociative path or an IER mechanism utilizing the waters in the other coordinating positions. The bridging oxygen exchange was thought to occur through the H+-

assisted reversible opening of the 0

0

0

ldo’ ‘do’ / \o/ 7 ring, breaking either one or both of the bridging bonds. The activation parameters for the bridging oxygens are AH* = 108.1(6.7) kJ M-i, AS* = 1.7(0.8) J M-’ K-l. At 0°C the calculated tilz for the bridging oxygens is about 3 years in 1.0 M acid. With respect to the [monomer12/[dimer] equilibrium value, it can be shown from the l aO data that K for the dissociation to the monomer cannot be greater than 4 x 10e6 at 40°C. This is in agreement with the essentially complete diamagnetic properties of MO(V) solutions in dilute acid media. Nothing firm is known about the exchange rate of the coordinated waters. Substitution for them with NCS- ” EDTA” or oxP2 is quite fast but, as we note 6th Mo(IV), this does not necessarily mean that the water exchange is equally rapid. However, at the present time all available evidence points to a relative high lability for the coordinated waters. Complexes of Mo,Oz+ The EDTA complex of Mo0% [Mo,O,(EDTA)]~is a well-studied, well-behaved ion. In water solution, 0.07 M, pH 4.67, at 80°C the maximum exchange on any oxygen in yl- or bridging positions after 4.25 days was 1%. In acetic acidacetate ion buffer at pH = 4.00 (other conditions the same) 83% of the yl-oxygen enrichment remains and essentially no bridging oxygen exchange occurs in the same time. In these experiments the yl-oxygens had 96% of the enrichment of the bridging oxygens as determined experimentally.

In strongly basic media only one type of oxygen exchanges, presumably the yl-oxygen. lg At 25°C k = 4 x 10m4 s-l at a pH of 12.96 and the only major term in the rate equation is first order in [OH-]. Considerable decomposition occurs during the kinetic experiments in basic media. Of all the complexes of Mo,Oq+, that of EDTA4is by far the most stable to exchanging either type of oxygen. In order to ascertain the effect of the organic bridge

Isotopic ‘*O exchange between solvent water and aquo molybdenum

2/o\y N/Mo\o/Mo\s I I n 4

4

L 0

0

/N

PH

0

R/[complex]

= k, + k,[OH-1.

At 25.0” C, ,u = 0.30 with LiClO,, k, = lO(3) x 10e4 s-l and k, = 5.1(3)x lo3 m-l s-l. Another rather startling difference is the salt effect. From Table 1 one sees only a small ionic strength effect while Table 2 shows a very large effect. For instance for the sulfur analogue at 25°C the half-time changes from 1.0 to 120 min on increasing the ionic strength from 0.033 to 0.50 with NaCl. While the direction of

Fig. 3. Change in I80 content of [Mo204(cysteine)J2with time: pH = 7.75 (a) or 6.05 (b) (buffered). Point T = 3000 min for a, or 6ooO min for b.

(21)

6.05 6.27 7.09 7.75 6.35

Fig. 2. [Mo,O,(cysteine)J-.

ion showed a relatively rapidly exchanging pair of oxygens which follow first-order behavior well and a much slower pair which seemed to be somewhat erratic (Fig. 3). The velocity of both reactions were mildly affected by pH changes. At 25°C and a pH of 6.0, kl was 5.5 x 10m4s- ‘, while at pH 7.5 it equalled 4.0 x 10m4 s-l (see Table 1). k2 was 3.6 x 10e6 and 1.6 x 10m5 s-l. In a stronger base, pH > 9 the rate of exchange had a positive [OH-] dependency.” The sulfur-bridged analogue [Mo,O,S,(I+ cysteine),12- gave values for the yl-rate constant of about half that of the oxygen-bridged complex under comparable, nearly neutral, conditions (Table 2). However, this is highly misleading since the oxygen- and sulfur-bridged complexes behave differently in other ways. For instance the pH dependence shows a maximum k at a pH of about 6.5 for the oxygen-bridged complex, while the sulfurbridged complex follows the rate equation (Fig. 4) :

491

Table 1. pH Dependence of the fast and slow exchange for [MozO,(cysteine),]*-”

0

between the two metal ions in Mo,O:+ the complex [Mo,O,(l+ cysteine)J2was studied (Fig. 2). This

ions

5.5 x 7.0 x 5.8 x 2.8 x 6.3 x

(S”-:,

10-4 10-4 1O-4 1O-4 1O-4

3.6 x 7.6 x 1.2 x 2.2 x 8.3 x

1O-6 1O-6 10-5 10-5 10-6b

’ [complex] = 0.040 M, Z = 0.50 M NaCl, buffer = 2,6lutidine_HCl except at pH 7.75 (2,4,6_trimethylpyridineHCl), pH f 0.10, temperature 25.00 + 0.03”C. ‘I = 0.68 M NaCl.

this effect is the same for the oxygen- and sulfurbridged complexes, the magnitude is not. The abnormal behavior of the bridging oxygen exchangefor [Mo204(cysteine)2]2- asshownin Fig. 3 is not understood. The increase in the rate of isotopic exchange shortly after the first type of oxygen has been exchanged suggested a chemical change which is autocatalytic and which also results in oxygen exchange. We have been trying to see ifthis effect is related to the catalytic effect of these solutions toward olefin hydrogenation. A study of the ion [Mo~O~(OX)~(OH~)~]~- was undertaken because it does not contain the additional bridge between the metal ions and also has a water in an equatorial position (Fig. 5). 180

Table

2. pH

effects on the oxygen [Mo,O,S,(cysteine),]*-”

Complex

pH

.016 .Oll .Oll .012 ,.012 .013 .017 .016 .018 .0155 .0155 .0155 .0155 .0155

6.05 5.91 6.19 6.75 7.18 7.46

Salt NaCl CsCl NaCl LiClO, N(propyl)41 NaNO, N(propyl),NO, LiClO, LiCIO, LiClO, LiClO, LiClO,

I 0.50 0.033 0.033 0.51 0.50 0.55 0.50 0.50 0.51 0.300 0.301 0.301 0.301 0.302

exchange

of

(Sic’)

6.4 x lo-” 2.4 x lo- 3b 1.1 x 10-2 9 x 10-S 9.4 x 10-5 1.5 x 10-4 1.7 x 10-4 1.9 x 10-4 3.0 x 10-4 1.5 x 10-4 1.9 x 10-4 3.6 x 1O-4 9.4 x 10-4 1.5 x 10-3

’ 25.O”C, unbuffered unless pH given, when 2,6-lutidine bulfer (0.15 M). bO”C.

G: D. HINCH et al.

01

0

I

I

I

1

2

3

[OH-]

(x 10’)

Fig. 4. Rate constants for O-exchange with [Mo,0,S,(cysteine),12-

exchange with this ion shows a single first-order plot corresponding to 4.0(l) oxygens and the other two oxygens exchange at least 1000 times slower. It was shown that the exchanging oxygens are yl-type and the coordinated waters and they cannot be distinguished from one another. In the pH region 3.8-5.1 the observed rate constant increased by about 50%. At a pH of 5.0 and 25°C the value of k was about 7 x low6 s-i. The activation parameters at pH = 5.05, [ox’- buffer] = 0.01 M, [complex] = 0.08 M, are AH* = 88.3(1.2) kJ M- ’ and AS* = -46.9(1.0) J M-’ K-i. At this point it may be useful to try to generalize the features which appear to determine the rate of yland bridge oxygen exchange when they are coordinated to MO(V). The latter appear in general to exchange very slowly. In either high acid or base the reaction proceeds more rapidly but even here the half-time is still in the many minutes to an hour range. Complexation of the positions normally occupied by water in the aquo ion by negative ions does not affect the bridging oxygen exchange rate strongly. However there is a suggestion that the coordination of a sulfur atom in a equatorial position does increase the bridging oxygen exchange rate probably due to a tram-effect. The yl-oxygens exchange more rapidly and are more affected by the other coordinated ligands and by solution conditions. While the rate of exchange is generally

0

0

2-

o__C/o+o\~“/oH2

p”‘I‘,‘IOlc/ ‘“\c=o

//“co

0

II

ij

Fig. 5. [Mo~O~(OX)~(OH,),]‘-.

at 25°C.

independent of acidity it is often sensitive to basic conditions. However this base sensitivity and the magnitude of the k, term seems to be related to water molecule(s) coordinated in one or more of the other octahedral positions. In the EDTA4complex, such a water is not available and so the rate is quite slow while with cysteine the carboxyl oxygen tram to the yl-oxygen is reversibly exchanged for water at a fairly rapid rate. For the oxalato complex neither the water nor the yl-oxygen are very rapid or very slow to exchange but their rates appear to be related in some way. We suggest that the application of the IER mechanisms gives a reasonable explanation of their experimental behavior. In it a proton transfer from a water to yl-oxygen followed by metal ion electronic rearrangement effectively exchanges the OH, to =O positions. If the (coordinated water)-isotopic exchange with solvent is rate-determining and the IER rate is roughly of the same order of magnitude or faster then the two types of oxygen atoms would appear to exchange at the same rate as is found. This approach also would suggest that any ligand which replaces water in either the equatorial or apical positions would lower the rate of yl-oxygen exchange, which is generally found. Mo(IV) The first conclusive evidence that the +4 oxidation state of molybdenum in aqueous solution was a trimer (Fig. 6) came from ‘so studies directed toward measuring the rates of oxygen exchange.“*” Again the main problem was finding a method of precipitating the anhydrous ion quickly from aqueous solution at low temperatures. While not ideal, tetramethyl ammonium thiocyanate gave a solid complex which served this purpose. The NCScomplexed very rapidly (and at the same time slowed down the exchange) and the (CH,),N+. salt of [Mo,04(NCS),15 - precipitated. Within 2 min at

Isotopic “‘0 exchange between solvent water and aquo molybdenum ions Type C water exchange was investigated following the loss in enrichment in

493 by

The ion was converted rapidly to

CMo~O&M*OW,12 -3

Fig. 6. Mo,Ot+(aq).

0°C the majority of the precipitation took place, too short at time for rearrangement to occur. With ‘*O enrichment in the solvent, at minimal contact times, the portion of enrichment transferred to the solid was less than 5% of that possible. Since the crystal structure of the precipitated solid” contained the Mo,O, core all of the oxygens (four) in that core had to exchange slowly in solution. That MO environment, shown in Fig. 6, contains two types of oxygen : one capping, and three bridging in addition to solvent water. Furthermore, it was possible to convert Mo,Of(aq) through this sequence :

whose crystal structure was determined as the easily precipitated [Pt(en),12+ salt. A count of the number of waters exchanging gave 3.0(2), suggesting that the exchanging waters were correctly identified. Of course, since the other six waters (type D) are replaced upon forming the complex no information about them can be obtained. The half-time for C type water exchange is about 1 h at 25°C in 1.0 M CH,SO,H. The reaction rate was not sensitive to light or [Mo30414+, increased slightly with ionic strength and was not affected by 10m4M Mo,O:+ or MOO:-. The rate constant was sensitive to pH and followed the rate law :

kbs= k, + k,JJCH ‘I.

The k,,,, term most likely occurs through hydrolysis of one of the D-type waters leading to a lowered charge on the ion and an increased rate constant. The activation parameters were nearly the same for both terms, being AH* = 100 kJ M-’ and AS* = 30 J M- ’ K- ’ (averaged). The high value for AS* suggests appreciable dissociative character leading to the activated state. + [Mo,O,(NCS)~]~-+ Mo,O:+ The effect of other complexing agents in the Dpositions on the exchange rates of the C waters was + [Mo~O~(OX)~(OH,),]~- + Mo,O:+ estimated for Cl-, NCS- and ox’-. With Cl- the + CMo,0,WN),15 -3 effect is a small decrease as Cl- increases while NCS with less than 4% core oxygen exchange with the is much more effective in lowering the rate. This is solvent. Even more remarkable, and a fact of probably due to its large association constants and the larger number of positions occupied for a considerable importance, is the transfer of Mo,Oi+ particular concentration. The tris-oxalato complex core oxygens through the sequence Mo,Oi+(aq) + Mo(hydroxide)l + Mo,Ot+(aq), where at least 96% had to be studied in less acidic media to retain the oxygen retention occurred. One usually expects coordinated oxalate ion and to prevent oxygen extreme base catalysis of oxygen exchange but exchange on the carboxylic acid groups. Actually apparently that only occurs for the waters not the there was a slight increase in rate constant with the oxalate complex but this is probably due in large part core oxygens. Type A oxygen (Fig. 6) exchanges at a rate to the large change in acidity. Thus D water significantly faster than type B. At 30°C the half-life is replacement by another ligand appears to cause a about 4 days in 1.OM acid and the number of oxygens lowering of k_,,, at least in the acid region. Efforts to measure the exchange rate for type D exchanged at this rate was 1.1(l). The rate was not affected significantly by small acidity (0.8-1.2 M) waters utilized [Mo(CN),14- as a precipitating agent. This reaction is not clean and neither the or [Mo,Oi+] (0.08-0.14 M) changes. Type B oxygen exchange (bridging) is very slow in reaction nor the product are ideal for this type of 1.1 M CH,SO,H. Only a fraction of the total experiment. Nevertheless, several runs gave halfexchange occurred in 534 days at 25°C. During this times of about 25(5) min at 0°C in 1.2 M acid. This is period, appreciable oxidation occurred to Ma(V) but remarkably slower than anticipated, especially since its presence did not cause rapid exchange. The half- the NCS-,23 0x2- and EDTA4- 24 complexes form life was estimated to be 5 + 2 years and is not highly much more rapidly. Ion pairing must be extremely sensitive to acidity. important in H20 replacement by anions.

494

G. D.

HINCH et al. an acetate buffer at a pH of 4.95 and electrolytic

Table 3. Oxygen type Protons A B

0 0

C D

2 2

Mo-O(av.) (A)

Approximate t,,,O [temperature (“C)]

2.020(3)

1 x 105(0)

2.916(7) 2.163(3) 2.26(2)b

2 x 108 (22) 2 x 105 (0) 1 x 103 (0)

“1 MH+.

bThis value comes from the SCN- complex structure.

The exchange rates under nominal conditions are compared with metal-oxygen distances in the solid state in Table 3. Oxygen transfer upon reduction of

Mo,Oi+(aq)

It has been conclusively demonstrated2”26 that Mo,Ot+(aq) can be reduced to two, well-defined, ions, Mo(IV)(III), and Mo(III),, which all evidence show to be trimeric. It is of interest to know if the Mo,Oi+ core remains intact in these ions. Toward this purpose the following sequence of reactions were carried outZ1 utilizing ‘*O enrichment in the core oxygens of Mo,Oi+(aq) : Mo,Ot+ -i Mo,O:+

+ Mo(IV)(III), or Mo(III), + [Mo~O~(OX)~(OH,),]~-.

Reduction was accomplished either electrolytically or with zinc metal at 20(1)“C with identical results. In all cases, greater than 93% and as high as 97% retention of all four oxygens was found. Furthermore in one experiment a solution of Mo(III), was allowed to exchange with solvent at room temperature for 2 months and no (f3%) exchange was detected. Clearly the trimeric structure including the core oxygens exists in these reduced ions and solvent oxygen does not replace or become equivalent to them over a long time even though protonation no doubt takes place. transfer Oxygen [Mo,O,(EDTA)]~ -

upon

reduction times of 3-24 h the core oxygen retention was 70-79(2)%. When carried out in enriched solvent with reduction times of 3-24 h, 75(2)% of the core oxygen was retained. This corresponds to 0.98(5) of four oxygens reaching isotopic equilibrium with the solvent. When using Zn(Hg) (for reduction), the isotopic oxygen transfer was constant with time for at least 29 h. Thus the reduced ion does not exchange ( f 3%) in that time. A significantly higher transfer of solvent oxygen did occur which corresponds to 1.68(5) oxygens, out of four, becoming equivalent to that in the solvent. The interpretation of these results is still tentative and relates to the intermediates (tetramers) formed in the oxidation back to the starting material and the tetrameric [M0(111)(1V),], formed in the Zn(Hg) reduction.

reduction

of

A similar study was made on the electrolytic and Zn(Hg) reduction products of [Mo,O,(EDTA)]~-. It has been shown27,28 that this produces [Mo,O,(OH)~(EDTA)]~and, when using Zn(Hg) as the reductant, an intermediate tetramer [Mo(III)(IV)], was observed. Oxidation back to the original complex was accomplished with 02. Using

Acknowledgements-We wish to express our appreciation to Guo Wei and the University Research Council.

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