Monophosphametallocenium cations of the cobalt group are easily prepared through metathesis of bis(η 5 -2,5-di-tert-butylphospholyl)lead ( 1) with [MCp*X 2 ] 2 (M ) Co, Rh, Ir). Crystal structure analyses confirm their metallocene structures, and electrochemical studies show that [CoCp*(PC 4 H 2 tBu 2 )] + (2) undergoes reversible reduction to a long-lived product at a potential of -0.74 V (vs SCE).

The reduction of the phosphacobaltocenium salt [CoCp*(2,5-PC(4)tBu(2)H(2))](+)[BPh(4)](-) (3; Cp*=pentamethylcyclopentadienyl) by magnesium in tetrahydrofuran (THF) furnishes the stable air-sensitive phosphacobaltocene [CoCp*(2,5-PC(4)tBu(2)H(2))] (4) in yields of up to 80 %. The crystal structure of 4 shows long Co-C(alpha) and short C(alpha)-C(beta)bonds 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-C(5)tBu(2)H(3))](+) [BPh(4)](-)(5) shows that replacing a CH group by an sp(2) phosphorus atom results in an anodic first reduction potential shift of 0.29 V.