Abstract
Abstract The equilibrium between the diamagnetic, planar nickel(II) macrocyclic complex known as NiCR2+(CR is 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2, 11,13,15pentaene) and its paramagnetic, six-coordinate dimethanol adduct has been examined in methanol solution as the tetrafluoroborate salt. From the temperature dependence of the electronic absorption spectrum, thermodynamic parameters of ΔH°46 = −4.35 kcal mol−1 and ΔS°46 = −8.72 cal deg−1 mol−1 were determined. From the excess ultrasonic absorption a relaxation frequency of 15 MHz was observed from which rate constants of k46 of 8.6 × 107 s−1 and k64 of 7.9 × 106 s−1 were calculated. The rate constant k46 is nearly the same as the rate constant for solvent exchange which had previously been determined by NMR. This implies that the solvent exchange is effected by the octahedral-planar equilibrium and that a two-step mechanism with a five-coordinate intermediate can be eliminated.
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