The potentially quadridentate ligand tris-(o-diphenylphosphinophenyl)phosphine (QP) reacts with various manganese(I) carbonyl compounds to give manganese(I) complexes in which the ligand acts as a bidentate, [MnX(CO)3(QP)], terdentate, [Mn(CO)3(QP)]+, or quadridentate, [Mn(CO)2(QP)]+, chelating agent. This has been substantiated by infrared studies and by the preparation and study of complexes of bis-(o-diphenylphosphinophenyl)phenylphosphine (TP) and of o-phenylenebisdiphenylphosphine (DP), which give compounds of the type [Mn(CO)3(TP)]+ and [MnX(CO)3(DP)], respectively. Manganese(II) complexes of QP could not be prepared either by oxidation of the manganese(I) complexes or by direct interaction of manganese(II) halides and the ligand. The preparation and infrared spectrum of cyanopentacarbonylmanganese(I), [Mn(CN)(CO)5], is reported.