Ionization and appearance potentials of cobalt carbonyl complexes

The ionization potentials of the compounds Co2(CO)6C2H2, Co2(CO)6C2(CH3)2, Co2(CO)6C2(CF3)2, Co2(CO)6C2(CH2Cl)2 and Co2(CO)6C2CH3C6H5 have been determined by electron impact measurements and found to show a good correlation with the Taft σ★ constants of the substituent groups on the ligand. The average ionic dissociation energies of the CoCO bonds are mainly dependent on the π acceptor ability of the ligand, and exhibit the sequence (CF3)2 < H2 < (CH3)2. Suggestions concerning the Coligand bond strengths are discussed in connection with the relative kinetic reactivities of such compounds in exchange and carbonyl group substitution reactions.