Low-spin complexes of Ni2+ with six NH3 and H2O ligands: A DFT-RX3LYP study

The gas-phase energy minimization of six-coordinate Ni2+ octahedral structures (S = 1) with H2O and NH3 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(NH3)n(H2O)m]2+·sH2O·(2 − s)NH3, 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 H2O, H-bonded to IS H2O, is changed to NH3, it deprotonates the latter with hydroxide strongly H-bonded to OS NH4+, a phenomenon not observed in the modeling of equivalent complexes of Zn2+ and Cu2+. 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 Cu2+, and much larger than for Zn2+ complexes. The stability of complexes with two OS H2O increases by 13 kcal mol−1 as IS H2O is replaced by NH3; this is twice as much as found for the analogous octahedral complexes. The deprotonation of IS H2O by OS NH3 leads to a stabilization by 10 kcal mol−1; interchanging IS H2O with OS NH3 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 ρbcp at the bond critical points of NiO and NiN bonds and a greater charge transfer to the metal (0.708-1.067 e).