THEORETICAL-STUDY OF THE ACTIVATION OF ALKANE C-H AND C-C BONDS BY DIFFERENT TRANSITION-METALS

The activation of C-H and C-C bonds by different transition metal atoms has been studied using quantum chemical methods including electron correlation. The metals studied are iron, cobalt, nickel, rhodium, and palladium. A general result for all these metals is the barrier for C-C insertion is found to be 14-20 kcal/mol higher than the barrier for C-H insertion. This can be explained by the difference in directionality between bonds to methyl groups and to hydrogen atoms. The size of the activation barrier is similar among transition metals in the same row but is considerably lower for the second-row metals than for the first-row metals studied here. This latter result follows from the more efficient sd-hybridization obtained for second-row metals, which in turn follows from the more similar size of the nd and (n + 1)s orbitals for these atoms. The differences in the atomic spectra between first- and second-row metals also play a part in making the barrier for second-row metals lower. Similar explanations can be used for the result that the exothermicity for the insertion reaction is larger for second-row metals and for the result that M-R1 and M-R2 bond energies are nearly additive in MR1R2 complexes of second-row metals.