Carbene complexes. Part 11. Steric and conformational effects upon the transition-metal reactivity of electron-rich olefins and derived species; heteroatom-donor-olefin–metal (Cr0, Mo0, W0, or RhI) and 2-imidazoline-N-donor–metal complexes, and the crystal and molecular structure of tetracarbonyl[NN′N″N‴-tetramethylbi(imidazolidin-2-ylidene)-NN″]-chromium(0)

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Abstract
The electron-rich olefin [:[graphic omitted]R]2, LR 2, (R = Me or Et), reacts thermally with (a)[Cr(CO)6], forming [Cr(CO)5LR][(1) or (2)] or cis-[Cr(CO)4(LR 2),][(3) or (4)] or(b)[Cr(η-C6H6)(CO)3], yielding [Cr(η-C6H6)(CO)2LMe], (5): [Cr(CO)5LMe], (1), forms the mixed ligand complexes cis-[Cr(AsPh3)(CO)4LMe], (6) or cis-[Cr(CO)4{C(OMe)Me}LMe], (7), upon treatment with AsPh3, or LiMe and then MeOSO2F, respectively. The related olefins [:[graphic omitted]Me]2(L′Me 2) and [:C(NMe2)2]2(tdae) do not afford carbene complexes with Cr0. From [M(CO)4(bhd)](M = Cr, MO, or W; bhd = bicyclo[2.2.1]hepta-2,5-dtene) each of the three olefins readily affords heteroatom-donor-olefln species [M(CO)4(olefin-NN″)]. For M = MO, the latter is transformed thermally into the N′N″-carbenemolybdenum complexes, only for olefin = LMe 2, but not for L′Me 2 or tdae. Attempted in situ syntheses of metal complexes of LH 2 or LH–LEt[from HC(OMe)2NMe2, and H2NCH2CH2NH2 or H2NCH2CH2NHEt, and a MO0 or RhI reagent] lead instead to 2-imidazoline [N[graphic omitted]H2](R = H or Et)N-bonded complexes of Mo0 and RhI. Carbon-13 n.m.r. (but also i.r. and 1H n.m.r.) spectra are diagnostic for differentiating the type of complex (carbene, olefln-N,N″, or imidazoline) and its stereochemistry. The crystal and molecular structure of [Cr(CO)4(LMe-NN″)] shows the metal to be in an approximately octahedral environment: the mutually-trans CO ligands are bent away from the olefin fragment and have Cr–C bond lengths of 1.90 Å compared to 1.82 Å of those cis; the CC bond length is 1.34 Å and there is probably little interaction with the Cr atom, the distance of closest approach being ca. 3.0 Å.

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