Metal Fragment Modulation of Metallacumulene Complexes: A Density Functional Study

Density functional calculations have been carried out on a series of metallacumulene complexes LmM(dC)nH2 with several MLm metal fragments to study the electronic structure, the bonding, and the reactivity of these complexes and how they are affected by the metal termini. The considered metal fragments include [(Cp)2(PH3)Ti], [Cp(PH3)2Mo]+, [(CO)5Cr], [(CO)5Mo], [(CO)5W], [Cp(dppe)Fe]+, [trans-Cl(dppe)2Ru]+, [Cp(PMe3)2Ru]+, [BzCl(PH3)Ru]+, [trans-Cl(PH3)2Rh], and [trans-Cl(PH3)2Ir], which are quite common in the chemistry of metal vinylidene, allenylidene, and higher cumulenes and range from a d2 to a d8 configuration and from electron-poor to electron-rich character. The optimized geometries calculated for the considered complexes have been found to be in good agreement with the available X-ray data and show that the peculiar carbon-carbon bond alternation superimposed to an average cumulenic structure, which is typical of these systems, is slightly perturbed by the nature of the metal fragment with the exception of the d4 [Cp(PH3)2Mo]+. Bonding energies have been calculated for all considered systems, and their dependence on the nature of the metal termini has been discussed. In particular an increase of the electron richness within d6 metal fragments causes a slight decrease of metal-cumulene bond energy. On the other hand, bond energies for d8 and, to a lesser extent, d4-d2 complexes are larger than those for the d6 analogues. The charge distribution and the localization of the molecular orbitals have been employed to explain the known reactivity patterns of this class of complexes and to forecast their variation with the nature of the metal fragment for both even and odd chains.