Localized molecular orbital studies of transition metal complexes. II: Simple π-accepting ligands

Localized molecular orbitals (LMOs) for several octahedral complexes are presented. Wavefunctions are calculated within the PRDDO approximations and localized by the Boys criterion. Complexes of general formula (NH₃)_x(CO)_6-xM, M = Cr⁰ or Mn⁺ and x = 1, 2, or 3 illustrate the general trends for carbonyl complexes. Weak to moderate π-bonding results in three equivalent inner shell LMOs dominantly of metal 3s, 3p and 3d character but highly delocalized to the carbonyls. These three LMOs flank the M-CO bond axis. Other π back-bonding situations result in metal-ligand double bonds which are nonequivalent and have σ-π separability [(NH₃)₅(py)Mn⁺] and also equivalent double bonds [(NH₃)₅(NO)Cr⁺].