Metal Complexes Co-ordination Number Four

The titles of this section have, in the interests of reasonable brevity, been somewhat oversimplified. For in this section we shall deal not only with inert complexes of co-ordination number four of undisputed tetrahedral stereochemistry, but also with some complexes and oxoanions of uncertain coordination number or stereochemistry and some complexes of undoubted lability. Thus, for instance, several reactions of molybdates and of vanadates are included in this section as they seem less inappropriate here than elsewhere in this Report. The arrangement of sections is by order of Periodic Table groups. 

Group V.—Vanadium. The kinetics of formation of [VOa(H2edta)] from vanadate and edta have been examined in acid (1.5 pH 2) aqueous solution, using stopped-flow techniques. The rate law is 

The most likely mechanism is that of a pre-equilibrium [V(OH)J+r [VO(OH)2]+ + HaO followed by bimolecular reaction with edta  

Rate constants were estimated for the forward and reverse reactions of equilibrium (1) and for the bimolecular step (2). The study of the reaction of vanadate with edta over a wide pH range is complicated by the existence of variously protonated and deprotonated forms of the reactants. Thus the overall kinetic scheme is complicated. The reaction of vanadate with alizarin (1,2-dihydroxyanthraquinone) is simpler, as there are only two reaction paths. These are the reactions of [VOa(OH)3]2 and of [VOa(OH)2] with the alizarin monoanion. 2 Formation reactions of are also important in some reactions 

The rate constants show some dependence on the nature of the cation present from the added MOH. This is ascribed to significant ion-association, which is not too surprising with a 6— anion, albeit a relatively large one. At a pH between 9 and 10, and at an ionic strength of 2.5 mol 1 (NaCl), the activation parameters are Aff = 28.3 0.5 kcal mol and AS = -I-17 2 cal deg mol . There is a marked similarity of the kinetic pattern here to that established for the paramolybdate ([Mo7024] ) anion.  

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Inert Metal Complexes Co-ordination Numbers Four and Five... 

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