The reaction of transition-metal dichlorides with 1,2-bis(dimethylphosphino)ethane (dmpe) leads to the brightly coloured, highly-crystalline octahedral complexes trans-MCl2(dmpe)2(M = Ti, V, Cr, or Fe). Although the Mn analogue could not be prepared, both trans-MnBr2(dmpe)2 and trans-Mnl2(dmpe)2 can be obtained from the respective dihalides. Alkylation of the MX2(dmpe)2(X = halide) compounds with either LiMe or MgMe2 leads to the dimethyl complexes trans-MMe2(dmpe)2(M = V, Cr, or Mn); the titanium complex isolated under these conditions is a mixture of trans-TiMeCl(dmpe)2 and trans-TiMe2(dmpe)2, while the iron species obtained is cis-FeMe2(dmpe)2. Magnetic susceptibility and e.s.r. measurements indicate a low-spin state for all the complexes except the manganese halide derivatives, and metal–ligand bond lengths observed in those complexes structurally characterised by X-ray crystallographic methods are consistent with the proposed ground-state configurations. There is a similarity between the electronic structures of the MCl2(dmpe)2 species and the metallocenes M(η5-C5H5)2, while the MMe2(dmpe)2 compounds resemble M(η5-C5Me5)2. Our studies indicate a progressive change in the nature of the M–Me groups upon lowering the electron count from 18 (Fe) to 14 (Ti). Thus, the titanium complex trans-TiMe1.3Cl0.7(dmpe)2 appears to contain a distorted methyl group involving a Ti ⋯ H–C interaction due to donation of C–H bond electrons into an empty orbital on the metal.
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Girolami et al. (1985) studied this question.