The reactions of P,O type ligands with the half-sandwich complexes [(η-C₅R₅)MCl₄] (R₅ = H₅, Me₅, ⁱPrH₄; M = Nb, Ta, W) have been investigated. Monodentate P-adducts were obtained with the β-amidophosphine Ph₂PCH₂C(O)NPh₂, whereas in the case of the keto ligand Ph₂PCH₂C(O)Ph a spontaneous HCl elimination occurred to give direct access to the corresponding phosphinoenolate complexes. The crystal structures of [(η-C₅H₅)NbCl₃{PPh₂CHC(O)Ph}], [(η-C₅H₅)TaCl₃{PPh₂CHC(O)Ph}] and [(η-C₅Me₅)TaCl₃{PPh₂CHC(O)Ph}] have been determined. Interestingly, the acetamido derived phosphine Ph₂PNHC(O)Me afforded O-adducts, which is an unusual bonding mode for a P,O ligand.