DECOMPOSITION OF (ALPHA-PHENETHYL)BIS(DIMETHYLGLYOXIMATO)COBALT(III) COMPLEXES - INFLUENCE OF ELECTRONIC AND STERIC FACTORS ON THE KINETICS AND THERMODYNAMICS OF COBALT CARBON BOND-DISSOCIATION

(α-Phenethyl)bis(dimethylglyoximato)cobalt(III) complexes were found to undergo reversible decomposition reactions according to C6H5CH(CH3)—Co(DH)2B ⇌ CoII(DH)2B + C6H5CH=Ch2 + 1/2H2 which attained measurable equilibria under ca. 1 atm of H2. The kinetics and, in some cases, equilibria of these reactions were determined for a series of complexes containing different trans-axial ligands B (pyridine (py), 4-CH3-py, 4-NH2-py, 4-CN-py, 2-CH3-py, 2-NH2-py, imidazole, 2-NH2CH2-py, 2-NH2CH2CH2-py, aniline, acetone). In the presence of the free-radical trap 2,2,6,6-tetramethyl-piperidinyl-l-oxy (Tempo), an additional pathway of decomposition of C6H5CH(CH3)-Co(DH)2py was observed that yielded the C6H5CH(CH3) radical-Tempo adduct according to C6H5CH(CH3)-Co(DH)2py + Tempo ⟶ CoII(DH)2py + C6H5CH(CH3)-Tempo Homolytic Co-C bond dissociation energies, Dco–c, were deduced from measurements of ΔH and ΔH* of these reactions and ranged from 17 kcal/mol for B = 2-NH2-py to 21 kcal/mol for B = 4-NH2-py. The variations in Dco–c and in the rates of Co–C bond homolysis were interpreted in terms of the electronic and steric influences of B. Thus, for constant steric influences, Dco–c increases with the electron-donor ability (pKa) of B whereas for constant pKa, Dco–c decreases with increasing steric bulk of B. The above reactions proceed through a common rate-determining step, namely, homolysis of the Co–C bond. © 1990, American Chemical Society. All rights reserved.