POSSIBILITY OF BOND-STRETCH ISOMERISM IN [CP(CO)2M]2(MU-E) COMPLEXES (M=MN, RE, CR AND W - E=S, SE AND TE) - A MOLECULAR-ORBITAL STUDY

Extended Huckel calculations have been carried out on [Cp(CO)2M]2(mu-E ) complexes (M = Mn, Re, Cr and W; E = S, Se and Te) in linear, bent a nd triangular geometries. The possibility of double well potential and consequent ''bond stretch isomerism'' in these complexes is analyzed. All the complexes with a 38 valence electron count (VEC) are calculat ed to have a minimum corresponding to the triangular geometry. The Mn and Cr complexes with a tellurido bridge have an additional minimum fo r the bent geometry; only the bent structure is known experimentally. The triangular isomers of these two complexes are marginally higher in energy, but separated by substantial barriers. The other M2E combinat ions studied adopt a triangular geometry. The difference in the behavi our between these complexes is explained by the fragment molecular orb ital approach and by considering the atomic radii of the M and E speci es. Single point ab initio calculations using the LANL1MB basis set on [cp(CO)2M]2(mu-E) (M = Mn and Re; E = S, Se and Te) confirm the relat ive energy orderings produced by the extended Huckel calculations. The relation between the M-E bond orders and bond lengths and the extent of Main Group d-orbital participation is also analyzed. It is predicte d that complexes with a VEC of 36 and 40 should adopt linear and bent geometries, respectively.