Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 56. Synthesis of iron–molybdenum compounds; crystal structures of [FeMo(µ-CR)(CO)6(η-C5H5)] and [FeMo2(µ3-RC2R)(CO)6(η-C5H5)2](R = C6H4Me-4)

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Abstract
Equivalent amounts of [Fe2(CO)9] and [Mo(CR)(CO)2(η-C5H5)](R = C6H4Me-4) react in Et2O at room temperature to give the dimetal complex [FeMo(µ-CR)(CO)6(η-C5H5)], the structure of which has been established by X-ray diffraction. Four CO groups are terminally bound to the iron atom and two to the molybdenum, with the latter also carrying the η-C5H5 ligand. The dimensions of the dimetallacyclopropene ring are Fe–Mo 2.823(1), µ-C–Fe 2.008(5), and µ-C–Mo 1.921(5)Å. Excess of [Fe2(CO)9] and [Mo(CR)(CO)2(η-C5H5)] react at room temperature in Et2O to give the trimetal compound [Fe2Mo(µ3-CR)(µ-CO)(CO)8(η-C5H5)]. In contrast, reaction between one equivalent of nonacarbonyldi-iron and two equivalents of the tolylmethylidynemolybdenum compound affords the 46-valence-electron irondimolybdenum cluster [FeMo2(µ3-RC2R)(CO)6(η-C5H5)2], the structure of which was confirmed by an X-ray diffraction study. The alkyne ligand adopts a µ3-(η2-⊥) bonding mode towards the metal triangle [Mo–Mo 2.764(1), Fe–Mo 2.732(1) and 2.761(1)Å], and one carbonyl ligand semi-bridges the molybdenum–molybdenum bond [Mo–C–O 162.5(3)°]. Reaction between one equivalent of the bis(cyclo-octene) iron complex [Fe(CO)3(η-C8H14)2] and two equivalents of [Mo(CR)(CO)2(η-C5H5)], in light petroleum at –40 °C, affords the thermally labile heptacarbonylirondimolybdenum compound [FeMo2(µ3-RC2R)(CO)7(η-C5H5)2]. The latter readily releases a molecule of CO to give [FeMo2(µ3-RC2R)(CO)6(η-C5H5)2], but this process is reversed at –20 °C. The spectroscopic properties of the new compounds are reported and discussed.

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