The ligands Ph2PCH2P(Se)Ph2 ( dpmSe ) and Ph2AsCH2CH2P(Se)Ph2 ( apeSe ) have been treated with Group 6 metal pentacarbonyl halo anions, [M(C0)5X]-, the Group 6 hexacarbonyls and the carbonyl halides M(C0)4X2 (M = Mo, W, X = C1, Br). Reaction of both dpmSe and apeSe with the anions [M(C0)5X]- in the presence of silver nitrate gave the complexes M(CO)5(L-L′) (L-L′ = dpmSe or apeSe ) with the ligand coordinated in a monodentate fashion through the Group 15 donor atom. Reaction of dpmSe with the hexacarbonyls gave only M(C0)4( dpmSe ) with the ligand chelated but, in contrast, apeSe could not be chelated to metal(0) and acted only as a monodentate ligand. These differences are rationalized in terms of the structure of the ligands. The behaviour of the ligands is much more similar in the metal(11) carbonyl halide chemistry. Both formed a mixture of two isomers of M(CO)3(L-L′)X2 (L-L′= dpmSe, apeSe ) in which the ligands are chelated . In the apeSe system, n.m.r. studies show these isomers are in an equilibrium which varies with temperature at a rate which is slow on the n.m.r. timescale, but the dpmSe complexes do not interchange. On reaction with further ligand, only one of the isomers of M(CO)3( dpmSe )X2 reacts to give incomplete formation of the dicarbonyl Mo(Co)2( dpmSe )2X2, which has one dpmSe ligand chelated and the other monodentate through phosphorus. On the other hand, for the apeSe system quantitative formation of Mo(CO)2( apeSe )2X2 was observed. For the apeSe system only, bubbling CO through a solution of M(CO)2( apeSe )2X2 gave M(CO)3( apeSe )2X2 with the ligands monodentate through arsenic. Extensive 31P and 77Se n.m.r. studies are described.