GEOMETRY OF COORDINATIVELY UNSATURATED 2-LEGGED PIANO STOOL COMPLEXESWITH 16 VALENCE-ELECTRONS - A THEORETICAL-STUDY

The structure of coordinatively unsaturated (16-electron) two-legged p iano stool complexes is analyzed with the extended Huckel methodology as well as with density functional theory. Pyramidal, and thus potenti ally chiral at the metal, geometries are predicted to be preferred for complexes bearing ligands like CO, CS, or NO+, containing low-lying p i-acceptor orbitals, as well as in the case of electropositive strong sigma donors, e.g., for silyl ligands. In all cases, however, the comp uted inversion barriers are low (<15 kcal mol(-1)). A minimum was loca ted for an eta(2)-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 ser ies [(eta(n)-CnHn)M(CO)(2)] (M = Fe, Mn, Cr, V; n = 4-7) is probed as a function of ring size and metal center.