Variable-temperature and -pressure proton nuclear magnetic resonance studies of the rates and mechanisms of N,N-dimethylformamide and acetonitrile exchange with the square-pyramidal complexes mono(solvento){3,7,11-tribenzyl-3,7,11,17-tetra-azabicyclo [11.3.1]heptadeca-1(17),13,15-triene}nickel(II)