Chemical and theoretical comparison between phosphido (X = PR2) and thiolato (X = SR) bent niobocenes of the type [Nb(cp)2L(X)][L = CO or P(OMe)3, cp =η-C5H5]. Structure of [Nb(cp)2(CO)(µ-SEt){Fe(CO)4}]

Abstract

The hydrido complexes [Nb(cp)₂H(L)][L = CO or P(OMe)₃, cp =η-C₅H₅] reacted with Mel to afford the iodo derivatives [Nb(cp)₂L(I)]1. The reaction of 1 with Na(SEt)led to the terminal thiolato complexes [Nb(cp)₂L(SEt)]2. Compounds 2 were used to bind the metal carbonyl fragments Fe(CO)₄ and M(CO)₅ from [Fe₂(CO)₉] and [M(CO)₅(thf)](thf = tetrahydrofuran) respectively. The corresponding monothiolato-bridged complexes [Nb(cp)₂L(µ-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 µ-SEt, µ-CO complex was formed. The new complexes 1–3 were characterized by IR, ¹H and ³¹P NMR spectroscopies. The X-ray analysis of [Nb(cp)₂(CO)(µ-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-Hückel molecular orbital (EHMO) conformational calculations carried out on the model complexes [Nb(cp)₂(CO)(PMe₂)] and [Nb(cp)₂(CO)(SMe)] are compared, and the imporatance of electronic and steric effects deducecd. An explanation for the non-formation of dibridged µ-SEt, µ-CO complexes is attempted on the basis of EHMO and X-ray results.