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Preparation of the dimeric starting compound [(C5H2 tBu3-1,2,4)Fe(μ-Br)]2 (1) gave a small amount of its green sodium bromide adduct [(C5H2 tBu3-1,2,4)Fe(μ-Br)2Na(DME)2] (2). Reduction of 1 under argon in the presence of diphenylacetylene yielded dark red [(C5H2 tBu3-1,2,4)Fe(μ-CPh)]2 (3) containing two bridging benzylidyne ligands resulting from diphenylacetylene scission. The dicobalt methylidyne complex [{(C5HiPr4)Co}2(μ-CH)(μ-H)] (5) was obtained in high yield from the reaction of [(C5HiPr4)Co(μ-Cl)]2 (4) with two equivalents of methylmagnesium chloride. According to DFT calculations the dinuclear alkylidyne complexes 3 and 5 contain d6 metal centers, namely, iron(II) (3) and cobalt(III) (5). Whereas the benzylidyne ligands of the iron complex 3 display an empty orbital of p z character at the bridging carbon atom, the p z orbital of the methylidyne carbon in the dicobalt complex 5 is filled with a lone pair of electrons.

The electron distribution within a mesityl ligand bridging a cyclopentadienyliron fragment attached to the mesityl π system and a second metal fragment connected to the ipso carbon of the mesityl ring has been probed using a copper(I) halide as a component in [CpmFe (μ,η5 :η1-C6H2Me3)CuCl] (2) and its copper bromide derivative 3. This approach minimizes steric effects and allows for DFT calculations the results of which are in very good agreement with structural data. The calculations show a significant carbene character already for the bare phenyl anion as a result of electrostatic repulsion of the lone pair pushing the π electrons away from the ipso carbon towards the para carbon atom of the aromatic ring. π Coordination of a cyclopentadienyliron(II) fragment to the phenyl anion to form a hypothetical sandwich complex does not change this situation. The iron center rather follows the unsymmetric distribution of π charge with an unsymmetrical coordination to the π system. Coordination of a copper(I) chloride moiety to the phenyl anion or to the hypothetical π complex [CpFe(C6H5)] in both cases equally lowers that carbene character by attracting the lone pair of the ipso carbon and thus decreasing its repulsive influence on the π electrons.

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