The cationic five-coordinated nickel(II) compounds with the tripod tetradentate ligands tris(2-diphenylphospinoethyl)amine, np₃, tris(2-diphenylarsinoethyl)amine, nas₃, and tris(2-diphenylphosphinoethyl)phosphine, pp₃, having the general formula [NiR(L)]BPh₄, L = np₃, nas₃, pp₃; R = CH₃, C₂H₅, CH₂C₆H₅, have been synthetized by reaction of the alkyl Grignard reagents with the five-coordinated complexes [NiX(L)]BPh₄, (L = np₃, nas₃, pp₃; X = halogen). The [NiCH₃(np₃)]BPh₄-CH₃COCH₃ complex has been studied by X-ray analysis using diffractometric data. The structure consists of [NiCH₃(np₃)]⁺ cations, of tetraphenylborate anions, and of interposed acetone molecules. The nickel atom is pentacoordinated, being bound to the four donor atoms of the ligand np₃ and to the carbon atom of the methyl group.