The potentially quadridentate ligand tris-(o-diphenylphosphinophenyl)phosphine (QP) reacts with chromium hexacarbonyl to give the complex [Cr(CO)₃(QP)], in which QP acts as a terdentate ligand. This can be converted into the complex [Cr(CO)₂(QP)] in which QP acts as a quadridentate ligand. Oxidation of the latter complex with mild halogenating agents gives the chromium(I) cation [Cr(CO)₂(QP)]⁺ which is then further oxidised to the chromium(III) complexes [CrX₃(QP)](X = Cl, Br), where QP acts as a terdentate ligand, and [Crl₂(QP)]l₃, where QP acts as a quadridentate ligand. The ligand bis-(o-diphenylphosphinophenyl)phenylphosphine (TP) also reacts with chromium hexacarbonyl to give the complex [Cr(CO)₃(TP)], but no complex could be isolated from its oxidation by mild halogenating agents. The preparation and properties of the complex [Cr(CO)₃(TAS)]{TAS = bis-(o-diphenylarsinophenyl)phenylarsine} are also described.