Chloro-, bromo-, iodo-, and trifluoromethylsulfonyloxy-(2,4,6-tri-tert-butylphenylimino)phosphines (MesNPX; X = Cl, Br, I, OTf) react quantitatively with 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Im) to give Lewis acid-base complexes with the general formula MesNP(Im)X. The dichlorophosphetidine (DippNPCl)(2) (Dipp = 2,6-diisopropylphenyl) represents a formal cyclodimer of an iminophosphine and reacts with Im to give a similar complex. The process represents a ligand induced dissociation of the phosphetidine framework and is reversed by the introduction of an appropriate Lewis acid. Solid state structures of RNP(Im)X complexes show that the closest contact between acid and base occurs between phosphorus and carbon in all cases, highlighting them as compounds that contain examples of C-->P coordinate bonds. Association of Im with phosphorus also effects a substantial increase in the P-X distance, but all derivatives maintain a short NP bond, indicating the presence of NP pi-bonding.