COMPARISON OF ISOMERIZATION RATES OF THE METAL-CARBONYL CLUSTER CP(ASTERISK)IRCP(2)CO(2)(CO)(3) IN 3 OXIDATION-STATES (47E, 48E, 49E) - DRAMATIC RATE ENHANCEMENTS IN THE ODD-ELECTRON SPECIES

The 48-electron cluster CpIrCp(2)Co(2)(CO)(3) has two known isomers, one with a terminal carbonyl ligand and two edge-bridging carbonyls (1 ) and the other with three edge-bridging carbonyls (2). The rate of th eir interconversion is dramatically dependent on the number of electro ns in the cluster. NMR studies establish that 2 is the thermodynamical ly favored isomer and that the isomerization is slow at ambient temper atures in the 48 e(-) complex (k(isom) approximate to 10(-6) s(-1) at 298 K). In contrast, isomerization proceeds very rapidly (k(isom)=400 s(-1)) through the 47-electron monocation as part of an efficient elec tron-transfer-catalyzed process. Cyclic voltammetry and square-wave vo ltammetry were used to measure the isomerization rate of the monocatio n. The catalytic nature of the anodically-induced isomerization was di agnosed by theoretical modeling of the electrode responses and by infr ared spectroelectrochemistry using a fiber-optic probe of an electroly sis solution. Reductions of the cluster isomers give 49-electron monoa nions. The anion 1(-) isomerizes over the period of a bulk electrolysi s to 2(-), setting the limits of k(isom) between 10(-1) and 10(-3) s(- 1) for 1(-). The relative rate of the cluster isomerization increases, therefore, in the order 48 e(-) much less than 49 e(-) much less than 47 e(-), with relative rates of 1:approximate to 10(4):10(8). Rate en hancements are rationalized in terms of changes in occupancies involvi ng redox orbitals either bonding or antibonding with respect to the tr imetallic framework. The results constitute a rare example of the dete rmination of a reaction rate through three oxidation states of a compl ex.