Solvent-exchange kinetics in nickel(II) solutions of aqueous tris(dimethylamino)phosphine oxide studied by pulsed phosphorus-31 nuclear magnetic resonance spectroscopy

Abstract

When small quantities of water are progressively added to solutions of nickel(II) perchlorate in anhydrous P(NMe₂)₃O electronic and n.m.r. spectroscopy provide evidence for the presence of the six-co-ordinate mixed complex [Ni{P(NMe₂)₃O}₂(OH₂)₄]²⁺ at a [H₂O] : [Ni²⁺] mol ratio of ca. 8 : 1. Phosphorus-31 relaxation times of such solutions have been measured at two frequencies (8 and 14 MHz) and five temperatures (8–45 °C). These data yield the ³¹P hyperfine coupling constant (A)= 17.5 MHz, the electronic relaxation times (T_1e and T_2e)= 3.91 × 10^–12 and 1.86 × 10^–12 s (at 25 °C and 14 MHz), the trace of the square of the zero-field splitting tensor (Δ)= 1.96 cm^–1, the relevant correlation time (τ_v)= 6.3 × 10^–12 s, and the kinetic parameters for the exchange bound P(NMe₂)₃O [graphic omitted] bulk P(NMe₂)₃O, i.e. k_M(25 °C)=(2.8 ± 0.4)× 10⁶ s^–1, ΔH‡= 25.5 ± 6.3 kJ mol^–1, and ΔS‡=–35.5 ± 16.7 J K^–1 mol^–1. The mechanism of ligand substitution is dissociative. Rates are higher than for identically substituted nickel(II) complexes by about three orders of magnitude. This result is compared with literature data, and accounted for by a lower activation enthalpy, by ca. 25 kJ mol^–1, as qualitatively expected from crystal-field theory.