Hydrogen–hydrogen bonding: A stabilizing interaction in strained chelating rings of metal complexes in aqueous phase

Two H⋯H close contacts, classically associated with steric hindrance, were found to be H–H closed-shell bonding interactions which stabilize a zinc(II) complex with nitrilotri-3-propanoic acid, [Zn(NTPA)(H2O)2]–, by ca. 11 kcal/mol in aqueous phase. The strain energy of three 6-member rings (ca. 40 kcal/mol) is significantly offset by these interactions the presence of which is indicated by the presence of a bond path, an elevated delocalization index, and a lowering of the atomic energy of each hydrogen atom involved in an H⋯H interaction by ∼4–5 kcal/mol. The difference between relevant bond path angles and geometrical bond angles in the 6-member coordination rings correlates well with strain energy and formation constants. Unexpectedly, the bite angle N–Zn–O is found to be the least strained.