An investigation of the rapid complexation of aquopentaamminecobalt(III) with molybdate in weakly basic solution

Abstract

The kinetics of equilibration of aquopentaamminecobalt(II1) with molybdate(V1) kf CO(NH,),OH,~+ + Co(NH,),MoO,+ + H,O have been studied at 25 OC and I = 1.0 M (NaC104) with 7.1 5 pH 5 8.0 by the stopped-flow method. The spectrophotometric equilibrium constant for (i), K I = kf/k,, is 475 f 15 M-I. The kinetics show a greater than first-order dependence on [Mood2-] and the second-order formation rate constant, kf, can be expressed as in k f = k , t kb[H+] t k,[M00,~'] t kd[H+][MOO,'-]Values of k, = 96 f 7 M-' SKI, kb = (1.1 f 0.2) X lo9 M-2 s-l, kc = (2.2 f 0.2) X lo3 M-2 s-l, and kd = (1.02 f 0.05) X loll M-) s-l are consistent with substitution at the Mo(V1) and not the Co(I1I) center. The paths k, and kb most probably correspond to the reaction of HM004-with Co(NH3)50H2+ (kl = 6.6 X lo4 M-I s-l) and Co(NH3)50H23+ (kz = 3.2 X 105 M-I SKI), respectively. These rate constants are much lower than those observed for addition of ligands to tetrahedral molybdate(V1) to give products of increased coordination number. The pathways involving two molybdate ions, k, and kd, correspond to reactions of dimolybdate(VI), a species which has not previously been detected in aqueous solution.The extreme inertness of the metal-oxygen bond in aquopentaamminecobalt(II1) results in very slow complex formation. With oxy anions which exhibit rapid oxygen exchange, complexation may be extremely rapid since the cobalt(III)-oxygen bond does not need to be broken. Studies on such reactions112 and those of other inert trivalent transition metal-aquo complexes3 have yielded valuable information on the nature of substitution mechanisms at oxy anion centers.Molybdate(V1) forms a series of stable octahedral complexes in weakly basic solution with bidentate ligands such as catech01,~ oxine,5 and oxinesulfonic acid,6 and kinetic studies of complex formation have been carried out. It is also known that molybdate(V1) complexes with C O ( N H~)~O H~~+ and that the product Co(NH3)5Mo04+ contains tetrahedrally coordinated molybdate(VI).' The fact that there is retention of tetrahedral geometry is of particular interest in this reaction. Conditions, [Mo(VI)] I 0.1 M and pH >7, were chosen so that isopolymolybdate species were not presentU8 Experimental Section Conditions. All measurements were carried out at 25 OC and I = 1.0 M (NaC104 or LiC104) in the presence of at least 0.1 M triethanolamine (TEA) buffer, unless otherwise stated.Materials. LiC104 was prepared and purified as described prev i o~s l y .~ NaC104 (Analar, Hopkin and Williams), triethanolamine (reagent grade, BDH), and Na2Mo04H20 (Analar, BDH) were not further purified. [Co(NH3)50H2](C104)3 was prepared by the standard method.I0J1 Measurement of pH. A Radiometer pHM4 meter fitted with G202C glass and K401 calomel (containing saturated NaCl instead of KCI) electrodes was used for pH measurement. The meter was calibrated at 1.0 M ionic strength with perchloric acid solutions (0.001-0.100 M) so that the measured pH corresponds to -log [H+], and pH used in this paper refers to such a quanti...

Kinetics of the interconversion of sulfido- and oxomolybdate(VI) species MoOxS4-x2- in aqueous solutions

“…6 Reference 18; /= 0.5 M (NaCICU, pH 7.1-8.0. biguities exist and the interpretation is not clearcut, in at least one case, that of the [H+]-dependent path in the reaction of Co(NH3)5H203+ with Mo042", it is clear that Mo04H" is involved. 18 With additional data obtained from H2180 exchange studies on Mo042', which also provide evidence for a fast [H+]-dependent exchange process incident at pH <11,19 the summary in Table V is obtained. Interpretation in terms of a reaction of Mo04H" with HS" (rate constant of 1.3 X 106 M"1 s'1) seems much more realistic than Mo042" with H2S (rate constant 4 X 102 M"1 s"1).…”