Synthesis and Properties of [CoCp*(2,5‐PC4tBu2H2)]: The First Monophosphacobaltocene

The reduction of the phosphacobaltocenium salt [CoCp*(2,5-PC4tBu2H2)]+[BPh4]− (3; Cp*=pentamethylcyclopentadienyl) by magnesium in tetrahydrofuran (THF) furnishes the stable air-sensitive phosphacobaltocene [CoCp*(2,5-PC4tBu2H2)] (4) in yields of up to 80 %. The crystal structure of 4 shows long CoCα and short CαCβbonds in the phospholyl ligand, consistent with a semi-occupied molecular orbital (SOMO) having a″ symmetry. A combined Amsterdam density functional (ADF)/photoelectron spectroscopic study, which confirms this assignment, gives ionisation energies (IE) of 5.02 eV from the SOMO and 8.89 eV from the phosphorus “lone pair”. A comparison of cyclovoltammograms for 3 and the corresponding cyclopentadienyl complex [CoCp*(1,3-C5tBu2H3)]+ [BPh4]− (5) shows that replacing a CH group by an sp2 phosphorus atom results in an anodic first reduction potential shift of 0.29 V.