The unusual dynamic solution behavior of two, five-coordinate Ni(II) complexes containing either the tridentate ligand bis(2-(dimethylarsino)phenyl)methylarsine (1) or bis(2-(dimethylarsino)phenyl)phenylarsin (2) and the bidentate ligand 1,2-phenylenebis(dimethylarsine) (3) was investigated by variable-temperature H-1 NMR spectroscopy in a variety of solvents. The preparations of 2 and [Ni(2)(3)1(ClO4)(2) (5) are reported. The solid-state structure of [Ni(1)(3)](ClO4)(2) (4) was reinvestigated. 4 4-4C(6)H(6) crystallizes in the monoclinic spacegroup P2(l)/c with a = 22.762 (6) Angstrom, b = 12.324 (2) Angstrom, c = 20,274 (5) Angstrom, beta = 108.42 (2)degrees, V = 5396 (2) Angstrom(3), Z = 4, and R/R(W) = 0.0587/0.0475 for 4210 data. Compound 4 adopts an unsymmetrical square-pyramidal geometry; the mean basal Ni-As distance = 2.285 (2) Angstrom and the apical Ni-As distance = 2.400 (2) Angstrom. Spectroscopic evidence suggests 5 has a similar structure. At low temperatures the spectral data for each complex are consistent with a rapid exchange between square-pyramidal enantiomers as one of the AsMe(2) groups of 3 oscillates between equivalent sites. At higher temperatures both AsMe(2) groups of 3 spin over the three positions of a facial site on the complexes. The bound tridentate ligands appear to be static. In certain solvents small amounts of free ligands are found, but they are not involved in the fluxional processes.