SYNTHESIS, REACTIVITY, AND SE-77 NMR-STUDIES OF THE ETA-2(SE)-SELENOPHENE AND ETA-1(SE)-SELENOPHENE COMPLEXES CP'(CO)2RE(SEL)

Selenophenes coordinate in CP'(CO)2Re(Sel) (Cp' = C5H5 or C5Me5; Sel = selenophenes) as a rapidly equilibrating mixture of eta-1(Se) and eta-2 isomers. Increasing the electron density on the Re by replacing Cp with Cp* favors eta-2-coordination, [GRAPHICS] while addition of methyl groups to the selenophene favors the eta-1(Se) isomer. These shifts in the eta-2 reversible eta-1(Se) equilibrium can be readily explained by noting that electron-donating methyl groups in the selenophene enhance the donor ability of the Se and reduce the pi-acceptor ability of the olefin which favors Se-coordination, while increasing the electron density on the Re strengthens pi-back-bonding to the eta-2 ligand. The uncoordinated selenophene Se atom in Cp*(CO)2Re(eta-2-Sel) (1) is capable of binding to M(CO)4(L)(THF) (M = Cr, Mo, W; L = CO or PPh3) to give dinuclear selenophene-bridged complexes CP*(CO)2Re(mu-2-eta-2(Se)-Sel)[M(CO)4(L)] in which the selenophene is eta-2 bonded to the Re and Se-bonded to a second metal (M). The reaction of 1 with Fe2(CO)9 gives three different types of compounds. One of them (11) results from Fe(CO)4 addition to the Se of the eta-2 isomer; another presumably results from coordination of Fe(CO)3 to the diene portion of the eta-1(Se) isomer. The third product has a thiaferrole-type structure in which a C-Se bond bas been cleaved. ''Se NMR studies of these different types of selenophene complexes are also reported.