TRENDS OF METAL-CARBON BOND STRENGTHS IN TRANSITION-METAL COMPLEXES

Two different types of trends of metal-carbon (M-C) bond strengths have been investigated for the sequence of second row transition metal atoms. In the first trend the bond strength dependence on the hybridization on carbon has been studied. It is found that sp hybridized carbon atoms form much stronger M-C bonds than sp(2) hybridized carbon atoms. The bonds formed by sp(3) hybridized carbon atoms are still weaker. The M-C bond strengths decrease markedly to the right in the periodic table. The second trend studied is the MC bond strength dependence for different alkyl groups. It is found that metal-methyl bonds are stronger than metal-ethyl bonds which in turn are slightly weaker than metal-propyl(n) bonds. The weakest metal-alkyl bonds studied here were found for bonds to isopropyl groups. The difference between the metal-alkyl bond strengths are largest to the left in the periodic table, and the difference essentially disappears to the right. All these trends of M-C bond strengths can best be explained by invoking ionic contributions in the M-C bonding and repulsive effects, depending on the number and type of ligands on the bonding carbon atom. The geometric structures show almost no signs of ct-bonding between the metal and carbon in the present systems, not even for the M-C2H systems where this might have been expected.