The Nature of the Phosphazene Nitrogen–Metal Bond: DFT Calculations on 2‐(Pyridyloxy)cyclophosphazene Complexes

Abstract

AbstractThe metal–nitrogen bond in six‐coordinate complexes [ML1Cl]+ (M = Co, Ni, Cu or Zn) of (pyridyloxy)cyclophosphazene, five‐coordinate complexes [ML1Cl2] (M = Cu or Zn) of green‐[NiL1Cl2], and dimetallic complexes [L2(CuCl2)2], [L3(CuCl)2]2+ and [L3(CuCl2)2] [L1 = hexakis(2‐pyridyloxy)cyclotriphosphazene, L2 = hexakis(4‐methyl‐2‐pyridyloxy)cyclotriphosphazene or L3 = octakis(4‐methyl‐2‐pyridyloxy)cyclotetraphosphazene] of red‐[NiL1Cl2], has been investigated by using density functional theory (DFT) and natural bond order (NBO) analysis. The calculations show that the divalent metal ions bind to the phosphazene ring nitrogen by a σ‐type bond and that the lengthening of the PN bonds, which flank the metal coordination site, can be explained as a result of electron density that is transferred from PN bonding orbitals to the 4s orbital of the metal ion rather than a decrease in the π component of the bond as suggested by earlier models. It can be assumed that this explanation of the bonding is valid for the wide range of metallo‐phosphazene complexes and that metal ions will bind to the PN backbone of polyphosphazenes in a similar manner.