Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 79. Synthesis and reactions of the alkylidynemetal complexes [M(CR)(CO)2(η-C5H5)](R = C6H3Me2-2,6, M = Cr, Mo, or W; R = C6H4Me-2, C6H4OMe-2, or C6H4NMe2-4, M = MO); crystal structure of the compound [MoFe(µ-CC6H3Me2-2,6)(CO)5(η-C5H5)]

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Abstract
The alkylidynemetal complexes [M(CR)(CO)2(η-C5H5)](M = Cr, Mo, or W, R = C6H3Me2-2,6; M = Mo, R = C6H4Me-2, C6H4OMe-2, or C6H4NMe2-4) have been prepared, and several of their reactions have been studied. New compounds characterised include [M{C(C6H3Me2-2,6)C(O)}(CO)(PR′3)(η-C5H5)](M = Mo, R′= Ph; M = W, R′= Me), [M(CC6H3Me2-2,6)(CO)(PR′3)(η-C5H5)](M = Mo, R′= Ph; M = W, R′= Ph or Me), and [MCo2(µ3-CC6H3Me2-2,6)(CO)8(η-C5H5)](M = Mo or W). However, the main study involved reactions of the alkylidynemetal compounds with [Fe2(CO)9]. The latter with [M(CC6H3Me2-2,6)(CO)2(η-C5H5)] yields the electronically unsaturated (32 valence electron) dimetal compounds [MFe(µ-CC6H3Me2-2,6)(CO)5(η-C5H5)]. In CO-saturated solutions, these products afford labile saturated (34 valence electron) complexes [MFe(µ-CC6H3Me2-2,6)(CO)6(η-C5H5)], which readily revert to the unsaturated species in the absence of CO. In contrast, reactions between [Fe2(CO)9] and [Mo(CR)(CO)2(η-C5H5)](R = C6H4Me-2 or C6H4OMe-2) afford the saturated dimetal compounds [MoFe(µ-CR)(CO)6(η-C5H5)]. However, the species with R = C6H4Me-2 in CH2Cl2 solution under a stream of nitrogen yields the labile unsaturated compound [MoFe(µ-CC6H4Me-2)(CO)5(η-C5H5)]. Moreover, an additional feature of the reaction between [Fe2(CO)9] and [Mo(CC6H4Me-2)(CO)2(η-C5H5)] is the formation of the trimetal compound [MoFe2(µ3-CC6H4Me-2)(µ-CO)(CO)8(η-C5H5)], a product produced in high yield if an excess of the iron carbonyl is used. The reaction between [Mo(CC6H4NMe2-4)(CO)2(η-C5H5)] and [Fe2(CO)9] gives a mixture of [MoFe(µ-CC6H4NMe2-4)(CO)6(η-C5H5)], [MoFe2(µ3-CC6H4NMe2-4)(µ-CO)(CO)8(η-C5H5)], and [Mo2{µ-C2(C6H4NMe2-4)2}(CO)6(η-C5H5)2]. The n.m.r. spectra of the various complexes are discussed. The molecular structure of the unsaturated compound [MoFe(µ-CC6H3Me2-2,6)(CO)5(η-C5H5)] has been established by an X-ray diffraction study [Mo–Fe 2.577(1), µ-C–Mo 1.976(2), and µ-C–Fe 1.861(2)Å]. Extended-Hückel molecular-orbital calculations have been carried out on model compounds, in an attempt to understand the nature of the bonding between the core M(µ-C)Fe atoms in the 32- and 34-valence-electron dimetal compounds. The results favour a dimetallacyclopropene description for the saturated complexes, and in affect a four-electron donation from the CM unit to the Fe(CO)3 groups in the unsaturated compounds. During the course of the work the complexes [WFe(µ-CC6H3Me2-2,6)(CO)4(PMe3)(η-C5H5)], [WFe(µ-CC6H3Me2-2,6)(µ-CO)(CO)2(PMe3)2(η-C5H5)], and [WFe(µ-CC6H3Me2-2,6)(µ-dppm)(µ-CO)(CO)2(η-C5H5)](dppm = Ph2PCH2PPh2) were also prepared.

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