A unusual bridging mode of vinylidene ligands in nonacarbonyl(vinylidene)dichromium complexes

Thermolysis of the pentacarbonyl(vinylidene)chromium complexes [(CO)5CrCC(R1)R2] (1a–d) (C(R1)R2=CMe2 (a), C(CH2)5 (b), C(Et)Me (c), C(tBu)Me (d)), affords binuclear, vinylidene-bridged nonacarbonyldichromium complexes, [(CO)5Cr(μ-η1,η2-CC(R1)R2)Cr(CO)4] (2a–d). The solid-state structures of 2a and 2d have been established by X-ray structural analyses. The structural data prove the μ-η1,η2-CC(R1)R2 bonding mode. To account for the NMR spectroscopic features a dynamic process is proposed involving rapid coordination site exchange of the CC bond between Cr(1) and Cr(2) (windshield wiper mechanism) and of one CO ligand.