Rate Dependence Upon Ligand Size

Because of the constriction provided by the occuli of cucurbituril (diameter 4 A), ligands which entirely fill the cavity (cf. Fig. 4) might be expected to exchange more slowly. A systematic investigation of this phenomenon is summarized in Table 2. The relevant numbers are the values of ki , the second-order rate constant for bimolecular formation of the complexes [11]. Although these can be measured directly by UV spectrophotometry for the slow incorporation of 

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It allows a detailed examination of the way in which the rate and mechanism depend upon the parameters of the metal ion, e.g. size, charge, electronic configuration, coordination number, and those of the solvent, e.g. size, dipole moment, nature of the donor atom. It can be applied to both labile and inert systems provided the solvated cations can exist in the conditions under which the measurements are made. For inert complexes it is possible to exchange a smaller number of ligands (ideally one) with the solvent and to extend the number of parameters that can be examined independently. The isotopic dilution technique, which requires that the time of sampling is short compared to the half-life of the exchange, has only been used in a limited number of cases. 

Studies reported in 1983 on the mechanism of CO substitution in metal carbonyl radicals have been continued. CO substitution by phosphines in V(C0)s is a second-order process with the rate of reaction dependent upon the basicity and size of the ligand. The kinetics and mechanism of 0- and N-Lewis base induced disproportionation of the same metal carbonyl and its phosphine substituted derivatives have also been examined, as have the mechanisms of redox reactions of V(C0)s with metal carbonylates. At low... 

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