A THEORETICAL-STUDY OF THE CHEMICAL BONDING IN M(CO)(X) (M=CR, FE, AND NI)

The equilibrium structure and bond energies of the transition metal complexes Ni(CO)(x) (x=1-4), Fe(CO)(5), and Cr(CO)(6) have been studied using the complete active space (CAS)SCF method and second-order perturbation theory (CASPT2). It is shown that the major features of the electronic structure are properly described by a CASSCF wave function based on an active space comprising the bonding; and antibonding orbitals directly involved in the metal-ligand bond. Remaining correlation effects are dealt with in the second, CASPT2, step. The computed energies have been corrected for basis set superposition errors (BSSE) and relativistic corrections have been added. Resulting bond distances and bond energies are in agreement with experimental data, when available: Cr(CO)(6), r(Cr-C)=1.91(1.91) Angstrom, D-0=148(153) kcal/mol; Fe(CO)(5), r(ax)(Fe-C)=1.79(1.81) Angstrom, r(eq)(Fe-C)=1.80(1.83) Angstrom, D-0=133(137) kcal/mol; Ni(CO)(4), r(Ni-C)=1.83(1.83) Angstrom, D-0=139(138) kcal/mol (experimental values within parentheses). Some excited states were computed for Fe(CO)(5). The first charge transfer (CT) state was located at 4.8 eV in agreement with an intense band found at 5.0 eV.