The coordination chemistry of the potentially semilabile tridentate ligand 2-pyridylbis(diphenylphosphino)methane (NPP) has been investigated. Bidentate (N, P) coordination occurs in CoCl2(NPP) (1) and [CdX(mu -X)(NPP)](2) (X = Cl (2); OAc (3)), prepared from the corresponding metal salts, in fac-Re(CO)(3)Br(NPP) (4) and in Fe(CO)(2)-(MA)(NPP) (6). The last is one of three products from the reaction of Fe(CO)(4)(MA) (MA = maleic anhydride) with NPP, the other two being Fe(CO)(3)(NPP) (7; P, P coordinated) and the unusual cyclic ylid Ph2PC(2-C5H4N)PPh2C(CH2CO2H)C(=O) (5). The ligand shows tridentate coordination in Cr(CO)(3)(NPP) (9), RuCl2-(PPh3)(NPP) (10), and possibly in PtCl2(NPP) (8). Carbon monoxide displaces one phosphorus arm of the ligand in 10. Anhydrous NiCl2 and NPP react in the presence of methanol to give NiCl2(P(OMe)Ph-2)(Ph(2)PCH(2)py) (12) in which the NPP ligand has been cleaved. This in turn reacts with O-2 to form trans-NiCl2(Ph2P(O)CH(2)py)(2) (13), The methine proton of NPP is transferred to the metal on reaction with Pt(C2H4)(PPh3)(2) and [Ir(COD)(NPP)]BF4 to form the hydride complexes Pt(H)(PPh3)(NPP-H) (14) and [Ir(H)(NPP)(NPP-H)]BF4 (15). In 15 the intact NPP ligand is tridentate. The structures of 1 - 7 and 12 - 15 have been determined.