The Origin of the MNXN Metallacycle Flexibility in the Chelate Iminophosphonamide and Amidinate Transition Metal Complexes

A series of square planar palladium and platinum iminophosphonamides [(η³‐allyl)M{Ph₂P(N‐p‐C₆H₄R)₂}] (1a–d, M = Pd; 2a, M = Pt), having N‐aryls of various electronic properties (R = OMe, Me, iPr, COOEt), have been synthesized and structurally characterized to reveal the correlation between the electronic properties of the N‐substituents and the geometry of the MNPN metallacycle. The experimental values of the MNPN bending angles and the pyramidalization at the N‐atoms were compared to the calculated ones for 1a–d and for the amidinate analogue [(η³‐allyl)Pd{PhC(N‐p‐C₆H₄Me)₂}] (7b). A thorough analysis of the orbital interactions for both palladium iminophosphonamide and amidinates have been carried out to elucidate the effect of the ligand's central atom (C, P) on the fluxional behavior of the MNXN metallacycle and the change of its electronic structure upon bending. The conclusions derived for palladium complexes are also supported by statistical analysis of the Cambridge Structural Database for other electronically saturated metal NPN and NCN complexes.