REACTIONS OF CO1-4+ AND CO4(CO)N+ WITH CYCLOHEXANE - C-H ACTIVATION AS A FUNCTION OF CLUSTER SIZE AND LIGAND SUBSTITUTION

Rate constants and product distributions for the bimolecular gas-phase reaction of cyclohexane with Co+, Co2+, Co3+, Co4(CO)n+ (n = 0-12), Ir2+, and Ir3+ are reported. All of the ions except Co2+ and Co4(CO)n+ (n = 2-4, 8-12) react with observable rates. In most cases the predominant product is C6H6 bound to the metal cluster. Observed products of the Co+ reaction and the failure of Co2+ to react agree with previously reported results. The results for Co4(CO)n+ are compared with previously reported results for Ir4(CO)n+ and Re4(CO)n+. Like the third-row clusters, the reactivity of the Co tetramer diminishes and disappears as ligands are added to increase the number of cluster valence electrons to more than 48. Co4(CO)7+ reacts slowly and the Co4(CO)8-12+ do not react at an observable rate. This can be rationalized on the basis of frontier orbital theory. It is found on examining the cluster molecular orbitals with extended Huckel calculations that the failure of Co2+ and Co4(CO)2-4+ to react can also be rationalized on the basis of frontier orbital theory. The contrast between the failure of Co4(CO)4+ to react and the reactivity of Co4(CO)5+ is attributed to a change from a high-spin electronic configuration of the former to a low-spin configuration for the latter.