Treatment of [Cr(CO)6] with C2(SR)4(LSR2) under photolysis in tetrahydrofuran affords cis-[M(CO)4(LSR2)](9; M = Cr, R = Me); analogues (9; M = Cr, Mo, or W; R = Et) have been obtained in better yield from the appropriate hexacarbonyl and C2(SEt)4 in EtOH in the presence of Na[BH4]. In contrast, the olefin (:[graphic omitted])2 and [Cr(CO)6] gave [Cr(CO)5{[graphic omitted][graphic omitted]}], which on heating reverts to the olefin and [Cr(CO)6]; a related complex [Cr(CO)5{C2(SEt)4}] can be obtained from [NEt4][Cr(CO)5Cl] and the olefin in the presence of [OEt3][BF4], but the Mo or W analogue was unstable, affording complex (9). Complexes fac-[M(CO)3(PPh3)(LSEt2)](10) have been prepared from [M(CO)3(NCMe)3] by successive treatment with C2(SEt)4 and PPh3 in toluene. Infrared [ν(CO)] and 13C n.m.r. spectra support the assigned configurations for complexes (9) and (10) and indicate some M–S π interaction; 1H n.m.r. spectra show two sets of inequivalent groups R in these complexes. These features are confirmed by a single-crystal X-ray analysis of the title complex, showing an approximately octahedral geometry around the metal, with Cr–S(av.)= 2.379(2), Cr–CO(trans to S)= 1.886(9), Cr–CO(trans to CO)= 1.833(7), and CC = 1.335(9)A. Crystals are monoclinic, a= 12.487(1), b= 21.838(2), c= 13.686(1)A, β= 117.32°, T= 21° C, space group P21/c, with two molecules per asymmetric unit. The structure was solved by direct methods and refined to R 0.049 by full-matrix least squares using 3 246 independent terms.