π-Complexes of transition metal tricarbonyls with cyclopolyenes and their boron analogs

The structures and stability of complexes of transition metal tricarbonyls (M = Co, Fe, Mn, Cr) with hydrocarbon and borane basal rings, (C _n H _n )M(CO)₃ and (B _n H _2n )M(CO) _n , respectively, as well as internal rotation of the metal tricarbonyl fragments in these systems were studied by the DFT B3LYP/6-311+G(df,p) method. Replacement of hydrocarbon basal fragments by isoelectronic borane rings leads to strengthening of the interaction between the apical and basal fragments, but does not change the trend in the heights of barriers to internal rotation. In all cases, the metal tricarbonyl fragment stabilizes the nonclassical planar form of the borane rings, whereas the complexes of metal tricarbonyls based on the classical nonplanar borane conformations are less thermodynamically stable.