Conductance measurements of the complex R,S,R,S-[NiL][ClO₄]₂(L = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane), in mixed solvents of N,N-dimethylacetamide (dma) and acetonitrile with nitrobenzene, which are of iso-relative permittivity, have shown that the first ion-pair formation constant (1 190 dm³ mol^–1) in nitrobenzene of much lower basicity decreases with increasing fraction of dma and MeCN, although MeCN is axially bonded to the nickel but dma is hardly co-ordinated. The axial co-ordination constant, K_NiS=[NiL(S)²⁺]/[NiL²⁺][S], of twenty donor solvents (S) in nitrobenzene as a non-co-ordinating diluent was also determined spectrometrically. It was found that the oxygen-donor co-ordinating solvents such as acetone and water for which Gutmann's donor number (DN) is less than 20 are not co-ordinated to the nickel, and the nitrogen-donor co-ordinating solvents with a nitrile group such as acetonitrile tend to be strongly co-ordinated in spite of their lower DN. These results show no correlation of K_NiS with DN and are discussed in terms of the π-acceptor properties of the C–N bond, steric hindrance due to the four methyl groups, and self-interactions of the donor solvents.