Transition Metal Complexes of a “Half-Parent” Phosphasilene Adduct Representing Silylene→Phosphinidene→Metal Complexes

The first [M(CO)(5)] complexes (M = Cr, Mo, W) bearing a donor-stabilized "half-parent" phosphasilene, in which the lone pair of the phosphorus atom coordinates to a metal center, have been synthesized and fully characterized. The latter complexes 2a (M = Cr), 2b (M = Mo), and 2c (M = W) result from conversion of the "half-parent" phosphasilene-DMAP adduct 1, LSi(dmap)=PH (DMAP = 4-N,N-dimethylaminopyridine; L = CH[(C=CH2)CMe(NAr)(2)]; Ar = 2,6-iPr(2)C(6)H(3)), with [M(CO)(5)thf] in 65- 72% yields. The somewhat unexpected reactivity of LSi(dmap)=(H)P: -> W(CO)(5) 2c toward the strong Lewis acid tris(pentafluorophenyl)borane was investigated. Strikingly, the borane does not abstract the DMAP donor. Most notable, the NMR spectroscopic and structural features of 2a-c, in accord with results by density functional theory calculations, clearly indicate that the already low Si=P pi-bond character in 1 is almost quenched through coordination of the [M(CO)(5)] group, suggesting that 2a-c rather represent push-pull donor -> silylene -> phosphinidene -> metal [DMAP -> Si(L):-> P(H):-> M(CO)(5)] than Si=P -> M complexes.