A series of compounds of the N 3 P 3 Cl (6 À n) (NHBu t ) n family (where n = 0, 1, 2, 4 and 6) are presented, and their molecular parameters are related to trends in physical properties, which provides insight into a potential reaction mechanism for nucleophilic substitution. The crystal structures of N 3 P 3 Cl 5 (NHBu t ) and N 3 P 3 Cl 2 (NHBu t ) 4 have been determined at 120 K, and those of N 3 P 3 Cl 6 and N 3 P 3 Cl 4 (NHBu t ) 2 have been redetermined at 120 K. These are compared with the known structure of N 3 P 3 (NHBu t ) 6 studied at 150 K. Trends in molecular parameters [phosphazene ring, P-Cl and P-N(HBu t ) distances, PCl 2 angles, and endo-and exocyclic phosphazene ring parameters] across the series are observed. Hydrogen-bonding motifs are identified, characterized and compared. Both the molecular and the hydrogen-bonding parameters are related to the electron distribution in bonds and the derived basicities of the cyclophosphazene series of compounds. These findings provide evidence for a proposed mechanism for nucleophilic substitution at a phosphorus site bearing a PCl(NHBu t ) group.