Effects of terminal substituents on metallacumulene complexes:: A density functional study on (CO)5Cr(=C)nX2 (X=F, SiH3, CHCH2, NH2, NO2)

Density functional calculations have been carried out on the series of metallacumulene complexes [(CO)(5)Cr(=C)(n)X-2)] (X = F, SiH3, CHCH2, NH2, NO2; n = 2-8) to study the effects of the terminal substituents on electronic structure, bonding, and reactivity of these complexes based on d(6) transition metal fragments. Optimized geometries have been calculated for all complexes and found in good agreement with the available X-ray experimental data. The calculated dissociation energies for the metal-cumulene bond are significantly affected mainly by the NH2 and NO2 substituents acting by resonance effect. In particular the pi-donor amino substituents cause a decrase while the pi-acceptor nitro substituents cause an increase of the bonding energies which are more evident for cumulenes with an odd or an even number of carbon atoms, respectively. The electronic structure has been analyzed in terms of the synergistic sigma donation pi back-donation model and the contribution from pi back-donation was found much more sensitive to the nature of the substituents. The perturbational theory of reactivity has been employed to explain the effect of the substituents on reactivity patterns of these complexes.