A series of complexes LM(CO)5[M = Cr, Mo, or W; L = Me2PCH2CH2PMe2(dmpe), Ph2PCH2PPh2(dpm), Ph2PCH2CH2PPh2(dpe), Ph2PCH2CH2CH2PPh2(dpp), or Ph2PCH2CH2AsPh2(ape)] has been prepared and characterised by analysis, and i.r., mass, and n.m.r. spectroscopy. The complexes can be methylated with Me3OBF4 to give [(MeL)M(CO)5]BF4. The acid-assisted nucleophilic substitution reaction used in the formation of the complexs can also be applied to their conversion into bridged complexes, L[M(CO)5]2. Variations in geminal complexes coupling constants 2JPCH and aromatic solvent induced shifts for the series of complexes(dmpe)M(CO)n(M = Cr, Mo, or W; n= 4 or 5) are discussed and compared with those in (Me2PXCH2)2(X = O or S) and [(Me3PCH2)2](BF4)2, whose syntheses are also reported. Changes in the 31P chemical shift are used to demonstrate that the chelation shift is comparable to the co-ordination shift. Kinetic studies of the rate of the chelation reaction, LM(CO)5→ LM(CO)4+ CO, have been used to elucidate details of its mechanism. The reaction follows first-order kinetics, as required for the intramolecular process. The magnitude of the enthalpy of activation (ca. 140 kJ mol–1) and the similarity in rate between phosphorus and arsenic nucleophiles, suggest a large dissociative component in the activation. The positive entropy of activation and particularly its large variation (+6 to +71 J K–1 mol–1) suggest a concerted process in the transition state. The smaller the potential chelate ring, the faster the reaction in the series L = Ph2P(CH2)nPPh2(n= 1, 2 or 3); this appears to be largely an entropy effect.