The low-spin, planar complex (Ph₂PS₂),Ni reacts with heterocyclic bases to form (Ph₂PS₂)₂NiB and (Ph₂PS₂)₂NiB₂ (B = pyridine or 4-methylpyridine). Average thermodynamic data for adduct formation in chlorobenzene at 30°C are: with pyridine K₁ 500, K₂ 251. mol^-1, ΔH₁° -34, ΔH₂° -44 kJ mol^-1; with 4-methylpyridine K₁ 1000, K₂ 50, ΔH₁° -35, ΔH₂°-41. With 2- methyl-pyridine only the 1 : 1 adduct is formed, with K 13, ΔH° -34. In these systems the five-coordinate 1 : 1 adducts are unusually stable and are shown to have the high-spin configuration.