Carbene complexes. Part 10. Electron-rich olefin-based mono-, bis-, and tris-carbene-tungsten(0) complexes and some derived six and seven-co-ordinate mono- and bis-carbene-dihalogenotungsten(II) and related molybdenum(II) species

Stable mono- and bis-carbene-tungsten(0) complexes [W(CO)5LR] or cis-[W(CO)4(LR)2][LR=:[graphic omitted]R (R = Me, Et, or CH2Ph)] are obtained by heating [W(CO)6] with the electron-rich olefin [:[graphic omitted]R]2, LR2. The six-membered chelate olefin [:[graphic omitted]Me]2, L′Me2, is less reactive, only forming [W(CO)5L′Me]. The cis-dicarbenetungsten(0) complexes are isomerised photochemically to the trans species, but readily revert thermally to the former. Bases react with (a)[W(CO)5LR] to yield cis-[W(CO)4(LR)Q](Q = PEt3 or PBun3), fac-[W(CO)3LRQ2][Q = P(OMe)3 or P(OPh)3], or {using successively LiR′(R′= Me or Ph) and [OEt3][BF4]}cis-[W(CO)4{C(OEt)R′}LR], or (b)cis-[W(CO)4(LR)2] to afford fac-[W(CO)3(LR)2Q][Q = C5H5N or P(OMe)3]. The tricarbene complex fac-[W(CO)3(LMe)3], obtained from [W(CO)3(η-mesitylene)], is rather unstable. The cis-dicarbene complexes cis-[M(CO)4(LR)2](M = Mo or W) with dihalogens give the metal(II) species [M(CO)2(LR)2X2](M = Mo, R = Et, X = I, Br, or Cl; M = W, R = Me or Et, X = I) and [M(CO)3(LR)2X2](M = W, R = Me or Et, X = I). The latter provides an example of a CO-carrying system, the interconversion of [W(CO)3I2(LR)2] and [W(CO)2I2(LR)2]+ CO being reversible as a function of temperature. The monocarbene complexes [M(CO)5LR](M = MO or W) behave differently, yielding respectively [ILR][Mo(CO)4I3] or [W(CO)4I2(LR)](R = Me). The frequency ν(CN2) is at 1 500–1 480cm–1for the carbene-tungsten(0), but at 1 530–1 500 cm–1 for the carbene-molybdenum(II) and -tungsten(II), complexes; values of ν(CO) are generally low, in accordance with the strong σ-donor character of the carbene ligand. The value of ΔG‡ for W–Ccarb. rotation in cis-[W(CO)4(LR)2] is ca. 10 kcal mol–1 and ca. 14 kcal mol–1 in [W(CO)4I2(LR)]. Carbon-13 n.m.r. chemical shifts for Ccarb. and CO in the tungsten(0) species are comparable, but distinguishable by 1H decoupling and 1H off-resonance studies, and are in the range 200–220 p.p.m. downfield from SiMe4. Values for 1J(183W–13C) and 2J(13C–31P) are provided.