SUBSTITUENT-EFFECT ANALYSIS IN ORGANOCOBALOXIMES - STUDIES OF THE FORMATION-CONSTANTS AND CYANO C-13 AND N-15 NMR CHEMICAL-SHIFTS OF ALKYL(CYANO)COBALOXIMES

Formation constants have been measured for a series of 16 alkyl(cyano)cobaloximes of the type YCH2Co(D2H2)CN-, where Y is an alkyl, aryl, heteroalkyl, or heteroatomic substituent. Cyano ligand C-13 and N-15 NMR chemical shifts are also reported for 17 (YCH2Co(D2H2)CN-)-C-13-N-15 complexes and for 14 cyano-bridged complexes of the type (YCH2Co(D2H2)CNCo(D2H2)CH2Y-)-C-13-N-15, which are formed when cyanide ion and cobaloxime are present at sufficiently high concentrations. These data, along with seven sets of literature data for various kinetic and equilibrium properties of YCH2Co(D2H2)L complexes, have been correlated using substituent-effect equations including only an inductive term, an inductive term plus a resonance term, or inductive, resonance, and steric terms. In every case, poor correlations using only an inductive term are markedly improved by inclusion of the steric term. In all but two cases, the correlations are significantly improved by inclusion of the steric term. In most cases, the resonance substituent effect is of comparable importance to the inductive substituent effect. The resonance effect can be interpreted in terms of hyperconjugation between the carbon-cobalt sigma-bond and the substituent. This concept is far more general than the concept of n --> sigma-donation from Y lone pairs to the Co-C sigma-bond, previously invoked by others, as it encompasses substituents without lone pairs and those which are resonance withdrawing as well as those which are resonance donating. By placing the substituent in the b position, such hyperconjugation is accentuated and the multiparameter substituent effect approach permits a quantitative assessment of its importance. The steric influence of Y is found to be significant and capable of being transmitted indirectly via weakening of the inductive effect due to decreased C-Co overlap. The possibility that such decreased Co-C overlap increases the resonance effect of Y by promoting hyperconjugation is discussed.