Structure, bonding, and reactivity in transition-metal-inserted thiophene complexes

Fenske-Hall molecular orbital calculations are reported for a number of structurally characterized transition-metal-inserted thiophene complexes. These complexes differ from one another in terms of metal center, ligands, coordination number, and metallacycle geometry. The results of the molecular orbital calculations demonstrate that many of these complexes are not only structurally but also electronically similar. Many of the complexes have a HOMO that is predominantly sulfur in character. This suggests a susceptibility toward electrophilic attack at the sulfur similar to that observed in (PEt3)(2)Pt[C,S'(S-(SC12H8)] and (triphos)Ir(H)[C,S-(SC8H6)]. The reactivity of a number of platinum- and (triphos)iridium-inserted complexes toward hydride addition is consistent with the fact that these are the only molecules that have a metal-based LUMO accessible to an attacking nucleophile. A combination of molecular orbital and molecular mechanics calculations suggests that the differences in metallacycle geometry observed in 18-electron complexes, planar and bent, arise primarily from steric rather than electronic factors. The delocalization of the metallacycle bonding which is observed in a few metal-inserted complexes is directly related to electron deficiency in the complexes and the orbital structure associated with specific coordination geometries.