DIRHENIUM COMPLEXES WITH S2CPR3 BRIDGES DONATING 4 AND 8 ELECTRONS - X-RAY STRUCTURES OF [RE-2(CO)(8)(MU-ETA(1)ETA(1)-S2CPCY3)] AND [RE-2(CO)(6)(MU-ETA(2)ETA(3)-S2CPCY3)]

The reaction of fac-[Re(CO)(3)(S2CPR3)Br] with [Re(CO)(5)](-) affords the dinuclear octacarbonyl compounds [Re-2(CO)(8)(mu-S2CPR3)] (3a-b), which have been isolated and characterized by spectroscopic methods. A n X-ray crystal-structure determination of the derivative 3a (R = Cy) shows that the S2CPCy3 ligand acts as an eta(1)(S);eta(1)(S') bridge d onating two electrons to each metal atom of a symmetrical (CO)(4)Re-Re (CO)(4) unit. This unit is presumably formed through a concerted proce ss involving carbonyl/sulfur exchange in the nonsymmetrical octacarbon yl (CO)(3)(S2CPR3)Re-Re(CO)(5) generated after the coupling of the bro mo derivative with the carbonyl anion. Complexes 3a-b can be obtained more directly by adding S2CPR3 to [Re-2(CO)(8)(mu-H)(mu-CH=CHnBu)], wh ich is formed by irradiation of Re-2(CO)(10) with 1-hexene. Hydride ad dition to the central carbon atom of the S2CPR3 ligand of 3a-b occurs with loss of PR3, to give the anion [Re-2(CO)(8)(mu-S2CH)](-) which ma y be isolated as a salt with the PPN+ cation. The octacarbonyl compoun ds 3a-b can be converted into hexacarbonyl complexes [Re-2(CO)(6)(mu-S 2CPR3)] (5a-b) by heating in toluene or octane at reflux temperature. According to an X-ray crystal-structure determination of the derivativ e 5a (R = Cy), the loss of one carbonyl ligand from each rhenium atom is accompanied by the rearrangement of the S2CPR3 ligand which changes its bonding mode from eta(1)(S);eta(1)(S') donating four electrons to eta(3)(S,C,S');eta(2)(S,S') donating eight electrons.