CHEMICAL AND THEORETICAL COMPARISON BETWEEN PHOSPHIDO (X=PR(2)) AND THIOLATO (X=SR) BENT NIBOCENES OF THE TYPE [NB(CP)(2)L(X)] [L=CO OR P(OME)(3), CP=ETA-C5H5] - STRUCTURE OF [NB(CP)(2)(CO)(MU-SET)(FE(CO)(4))]

The hydrido complexes [Nb(cp)(2)H(L)][L = CO or P(OMe)(3), cp = eta-C5H5] reacted with Mel to afford the iodo derivatives [Nb(cp)(2)L(I)] 1. The reaction of 1 with Na(SEt) let to the terminal thiolato complexes [Nb(cp)(2)L(SEt)] 2. Compounds 2 were used to bind the metal carbonyl fragments Fe(CO)(4) and M(CO)(5) from [Fe-2(CO)(9)] and [M(CO)(5)(thf)] (thf = tetrahydrofuran) respectively. The corresponding monothiolato-bridged complexes [Nb(cp)(2)L(mu-SEt){M(CO)(n)}] 3 (M = Fe, n = 4; M = Mo or W, n = 5) were obtained only in the case of L = CO and no dibridged mu-SEt, mu-CO complex was formed. The new complexes 1-3 were characterized by IR, H-1 and P-31 NMR spectroscopies. The X-ray analysis of [Nb(cp)(2)(CO)(mu-SEt){Fe(CO)(4)}] showed an endo position of the ethyl group with respect to the CO ligand bound to the niobium. The results of extended-Huckel molecular orbital (EHMO) conformational calculations carried out on the model comlexes [Nb(cp)(2)(CO)(PMe(2))] and [Nb(cp)(2)(CO)(SMe)] are compared, and the importance of electronic and steric effects deduced. An explanation for the non-formation of dibridged mu-SEt, mu-CO complexes is attempted on the basis of EHMO and X-ray results.