Geometry of Coordinatively Unsaturated Two-Legged Piano Stool Complexes with 16 Valence Electrons:  A Theoretical Study

The structure of coordinatively unsaturated (16-electron) two-legged piano stool complexes is analyzed with the extended Hückel methodology as well as with density functional theory. Pyramidal, and thus potentially chiral at the metal, geometries are predicted to be preferred for complexes bearing ligands like CO, CS, or NO+, containing low-lying π-acceptor orbitals, as well as in the case of electropositive strong σ donors, e.g., for silyl ligands. In all cases, however, the computed inversion barriers are low (<15 kcal mol-1). A minimum was located for an η2-coordination of an acyl ligand in [CpFe(acyl)NO]+, which may account for the stereospecific ligand substitution in acyl-containing systems. The ground-state geometry of the isoelectronic series [(ηn-CnHn)M(CO)2] (M = Fe, Mn, Cr, V; n = 4−7) is probed as a function of ring size and metal center.