Low-spin complexes of Ni 2+ with six NH 3 and H 2O ligands: A DFT–RX3LYP study

Abstract

The gas-phase energy minimization of six-coordinate Ni 2+ octahedral structures ( S = 1) with H 2O and NH 3 using the DFT/RX3LYP/6-311++G(d,p) level of theory and S = 0 leads to the formation of square planar complexes of the type [Ni(NH 3) n(H 2O) m] 2+·sH 2O·(2 − s)NH 3, n + m = 4, 0 ≤ n, m ≤ 4, 0 ≤ s ≤ 2. The H-bonding between inner- and outer-sphere (IS and OS) ligands results in the formation of six-member rings characterized by ring critical points. If OS H 2O, H-bonded to IS H 2O, is changed to NH 3, it deprotonates the latter with hydroxide strongly H-bonded to OS NH 4 +, a phenomenon not observed in the modeling of equivalent complexes of Zn 2+ and Cu 2+. The four-coordinate hexa complexes are found to be over 50 kcal mol −1 more stable than the equivalent octahedral structures; this is twice as large as reported for Cu 2+, and much larger than for Zn 2+ complexes. The stability of complexes with two OS H 2O increases by 13 kcal mol −1 as IS H 2O is replaced by NH 3; this is twice as much as found for the analogous octahedral complexes. The deprotonation of IS H 2O by OS NH 3 leads to a stabilization by 10 kcal mol −1; interchanging IS H 2O with OS NH 3 increases stability by a further 4–8 kcal mol −1. The increased stability of these four-coordinate hexa complexes is associated with greater charge density ρ bcp at the bond critical points of Ni O and Ni N bonds and a greater charge transfer to the metal (0.708–1.067 e).