TRIALKYLPHOSPHINE CARBON-DISULFIDE ADDUCTS AS 8-ELECTRON BRIDGING LIGANDS - X-RAY STRUCTURES OF [MN2(CO)6(MU-S2CPCY3)] AND [MN2(CO)4(MU-S2CPCY3)(MU-DPPM)]

Carbon disulfide adducts of bulky trialkylphosphines, such as PCy3 or PPr(i)3, react with Mn2(CO)10 to give dimanganese hexacarbonyl complexes [Mn2(CO)6(S2CPR3)]. An X-ray determination of the structure of the derivative with S2CPCy3 (1a, triclinic, space group P1BAR, a = 11.652 (4) angstrom, b = 11.368 (3) angstrom, c = 10.973 (3) angstrom, alpha = 85.76 (2)degrees, beta = 77.29 (2)degrees, gamma = 86.00 (2)degrees, Z = 2, R = 0.061, R(W) = 0.064) showed that the ligand adopted a new bonding mode, which can be described as eta-2(S,S') chelate to one manganese and eta-3(S,C,S') pseudoallyl to the other manganese, thus bridging unsymmetrically the Mn-Mn bond and donating 8 e to the metals. The analysis of the bond distances within the Mn2S2C core suggests a strong interaction between the S2C group and the metal atoms. This is confirmed by the high thermal stability of the 8e bridge, which permits the preparation of a variety of derivatives [Mn2(CO)5(L)(mu-S2CPR3)] and [Mn2-(CO)4(L)2(mu-S2CPR3)] by carbonyl substitution reactions requiring high temperatures. Thus, the thermal robustness of the 8e bridge has been proved by the X-ray structure determination of the derivative [Mn2(CO)4(mu-dppm)(S2CPCy3)] (4e), which has to be prepared by heating the parent hexacarbonyl (1a) and dppm in refluxing xylene for 30 h. (4a is triclinic, space group P1BAR, a = 16.403 (4) angstrom, b = 14.927 (3) angstrom, c = 11.903 (3) angstrom, alpha = 116.48 (3)degrees, beta = 106.46 (3)degrees, gamma = 81.49 (2)degrees, Z = 2, R = 0.039, R(W) = 0.044). C-13 NMR spectra display the signal of the central carbon of the S2CPR3 ligand in the range delta-79.19-85.51 ppm, being therefore a very useful tool for the characterization of this type of bridge. A close inspection of the structures suggests an alternative view of the bonding, consisting of a nearly planar eta-4-activated-Mn-S-C-S ring which donates (formally) 5 electrons to the second manganese atom. A comparison of the structural parameters of the molecules with those known for cymantrene, cymantrene-like, and eta-3-allyl compounds is given.