BONDING AND REACTIVITY IN TRANSITION-METAL COMPLEXES CONTAINING THIOPHENIC LIGANDS

The results of Fenske-Hall molecular orbital calculations are reported for transition metal complexes containing thiophenic ligands. These ligands interact with the metals in several different binding modes, and complexes of Cr, Mn, Fe, Ru, Rh, and Ir which incorporate eta5, eta1, eta4, and ring-opened thiophenic ligands have been studied. Although metal-ligand interactions in eta5-thiophene complexes are very similar to the interactions in metal cyclopentadienyl (Cp-) complexes, thiophene is a poorer electron donor but more effective electron acceptor than Cp-. In the eta5 complexes, metal-thiophene binding is strongest when the acceptor ability of thiophene becomes important. The pyramidal bonding which is always observed around the ring sulfur atom in eta1(S)-bound thiophene and dibenzothiophene (DBT) complexes is related to the need for the ligand to act as a two- (and not four-) electron donor. The slightly better donor ability of DBT, when compared with thiophene, appears to be responsible for the greater stability of the Fe-(eta1-DBT) complex. The unusual eta4 and ring-opened complexes Cp*Ir(eta4-2,5-Me2T) and Cp*Ir(C,S-2,5-Me2T) are related by a formal oxidative addition. The electronic structures of these complexes suggest that a very electron rich metal center is a necessary precursor for insertion of the metal center into the C-S bond of the thiophene ring.