Carbene complexes. Part 21. Synthesis and characterisation of bis(carbene)molybdenum(II) complexes and dimetal(0) complexes of the Group 6 elements containing novel bridging bis(carbene) ligands; X-ray structures of [Mo(CO)2(LEt)2(OSO2CF3)2][LEt=CN(Et)(CH2)2NEt] and [W(CO)5{C(OEt)CH2C6H4CH2C(OEt)-o}W(CO)5]

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Abstract
Treatment of the bis(carbene)tetracarbonylmolybdenum (0) complex cis-[Mo(CO)4(LEt)2][LEt= [graphic omitted]Et] with two equivalents of silver trifluoromethanesulphonate in tetrahydrofuran (thf) readily yields the molybdenum(II) complex [Mo(CO)2(LEt)2(OSO2CF3)2](1). Reaction of the complexes [M(CO)6](M = Cr or W) with Mg[(CH2)2C6H4-o](thf)(M = Cr) or o-C6H4(CH2MgCl)2(M = W) in thf affords the crystalline yellow µ-bis(carbene)-dimetal(0) complex [{M(CO)5[C([graphic omitted]g(thf)n][M = Cr, n= 3 (2a); M = W, n= 4 (2b)], which when extracted into water and treated with [Et3O][BF4] furnishes the appropriate orange µ-bis(carbene) bimetallic complex [[graphic omitted](OEt)CH2C6H4CH2[graphic omitted](CO)5][M = Cr (3a) or W (3b)]. Complex (3a) with an excess of PEt3 gives the red-orange complex [{[graphic omitted](OEt)CH2C6H4CH2[graphic omitted](CO)4(PEt3)}]. Reaction of [M(CO)6] with o-C6H4[CH(SiMe3)Li(tmen)]2[tmen = Me2N(CH2)2NMe2] yields [[graphic omitted]OLi(OEt2)(tmen)]CH(SiMe3)C6H4CH SiMe3)[graphic omitted](CO)5]. In the crystalline bis(carbene)-molybdenum(II) complex (1), the Mo atom resides at the centre of a severely distorted octahedron [Ccarb–Mo–Ccarb 134.9(2)°], with the two five-membered LEt rings arranged so as to be approximately parallel, and mean lengths Mo–Ccarb 2.154(5), Mo–CO 1.961(6), and Mo–O 2.177(4)Å. In the crystalline ditungsten(0) complex (3b), each W is in an octahedral environment and selected mean parameters include W–Ccarb 2.155(12), W–CO 2.03(4), and Ccarb–O 1.317(14)Å.

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