THE QUESTION OF METAL METAL-BONDING IN EDGE-SHARING BIOCTAHEDRAL MO(III) COMPLEXES - VARIABLE TEMPERATURE H-1-NMR STUDY OF MO2CL6(PMEXET3-X)4 (X = 0-3) AND THE MECHANISM OF THE FACE-SHARING TO EDGE-SHARING TRANSFORMATION

Homoleptic (L = L') and heteroleptic (L not-equal L') edge-sharing bioctahedral Mo(III) complexes of formula Mo2Cl6L4-nL'n (n = 0-2; L, L' = PMexEt3-x; x = 0-3) have been generated in solution and investigated by paramagnetic H-1 NMR spectroscopy. The derivatives have been obtained by interaction of a face-sharing bioctahedral Mo2Cl6L3 precursor with L' or, for the case of n = 0, L = L' = PMeEt2, by brief reflux of MoCl3(THF)3 and 2 equiv of the phosphine in toluene. The Mo2Cl6L3/L' interaction occurs regioselectively to afford a single product, and this is followed by other processes, one of them being ligand exchange to generate a single stereoisomer of formula Mo2Cl6L2L'2. A relatively simple mechanistic scheme which is based on the previously proven higher trans effect of phosphine ligands with respect to chlorides for pseudooctahedral Mo(III) coordination compounds allows a rationalization of all stereochemical results. The paramagnetic shift of the phosphine proton resonances for the edge-sharing bioctahedral Mo2Cl6L4-nL'n compounds increases as the Me/Et ratio for the phosphine substituents decreases, and it is found in general that, for a given Me/Et ratio, the system is more paramagnetic when more ethyl groups are located on equatorial phosphines. For instance, Mo2Cl6(PEt3)2(ax-PMe3)2 is more paramagnetic than the isomeric Mo2Cl6(PMe3)2(ax-PEt3)2. Variable temperature H-1 NMR measurements have been carried out for Mo2Cl6(PMexEt3-x)4 (x = 0-3) and for Mo2Cl6(PEt3)4-n(ax-PMe3)n (n = 1, 2). These investigations allow considerations on the ground electronic structure of these materials to be made.