Synthesis, structural characterisation and electrochemical studies of neutral alkenylcarbyne tungsten complexes bearing chelating bidentate and tridentate phosphanes

Neutral alkenylcarbyne complexes [X = SCN; n = 1 (2a), 4 (2b). X = OCN; n = 1 (2c), 4 (2d)] were prepared by reaction of the acetonitrile complexes [n = 1 (la), 4 (Ib)] with M[OCN] or M[SCN] (M = Na, K, Bu4N), and were fully characterised. The synthesis of complex 3, containing the tridentate phosph ane triphos [triphos = PPh(CH2CH2PPh2)(2)], is also described. The structur es of 2b and 3 were determined by Xray diffraction methods. In both complex es, the coordination of the W atom was octahedral, with the N-bonded thiocy anate (2b) or the chlorine (3) anions trans to the alkenylcarbyne group. Th e W-N and W-Cl bond lengths were significant longer than the average found in the literature; this indicates the remarkable trans influence of the alk enylcarbyne moiety. The electrochemical behaviour of complexes 2a-d and 3, as well as that of the related compounds D (2e: X = E n = 4), (2f: X = Cl, n = 1), (2g: X = S2P(OEt)(2), n = 1) and A (4: n = 1) was investigated by c yclic voltammetry (CV) and controlled potential electrolysis (CPE) in aprot ic media and at a Pt electrode. They underwent irreversible anodic and cath odic processes, the former usually being multi-electronic in the time scale of CPE and involving anodically induced proton dissociation from the alken ylcarbynes. These ligands behave as remarkably strong x-electron accepters and from the linear relationship between the oxidation potential of the fir st anodic wave and the electrochemical P-L ligand parameter, it was possibl e to estimate, for the first time for a carbyne-containing metal centre, th at is, the site {(dppe)(CO)(2)W drop C-R} the values of the electrochemical electron richness (E-s) and the polarisability (beta) parameters. These va lues indicate that this site has low electron richness and low polarisabili ty; this is accounted for by the extensive pi-electron acceptance of the al kenylcarbyne ligand. P-L was also estimated for the S2P(OEt)(2)(-) ligand, which was shown to act as a rather strong net electron releaser.