Several series of platinum(II), palladium(II), and nickel(II) complexes bearing 8-(diphenylphosphino)quinoline (PQ(H)) or its 2-methyl or 2-phenyl derivatives (PQ(Me) or PQ(Ph)) were synthesized, and their crystal structures and behaviors in solution were investigated. Most of the complexes [M(PQ(R))(2)]X-2 (M-II = P-II, Pd-II, or Ni-II; R = H, Me or Ph; X = monoanionic ions) characterized in this study have an approximately square-planar coordination geometry with two bidentate P,N-chelating or monodentate P-donating quinolylphosphine ligands in the cis(P,P) configuration. A large steric requirement from the Me or Ph substituent introduced at the 2-position of the quinoline ring gives the resulting complexes severe distortion. The Pt-II and Pd-II complex cations maintained the square-planar coordination geometry, but the M(II )center was displaced from the chelating ligand plane. This bending of the chelate coordination makes the M-N(quinoline) bond weaker, as demonstrated by the longer M-N bonds. In accord with the bond weakening, the partial dissociation of the PQ(H) or PQ(Me) chelates by substitution with halide anions were observed using UV-vis spectroscopy and X-ray crystallography. In contrast, the PQ(Ph) complexes were stable in solution toward the addition of halide anions; the intramolecular pi-pi stacking interaction between the coordinating quinolyl and the 2-substituted phenyl rings protects the M-II center from nucleophilic attack. In the corresponding Ni-II complexes, the steric congestion arising from the mutually cispositioned PQ(R) ligands resulted in a larre tetrahedral distortion around the Ni-II center. However, the intramolecular pi-pi stacking interaction was still effective in the PQ(Ph) complex, and this interaction can explain some unusual robustness and electrochemical properties of the Ni-II-PQ(Ph) complex.