The five-coordinate palladium(II) and platinum(II) complexes [M(PP3)CI]CI [M = Pd (1), Pt (2)] (PP3 = tris[2-(diphenylphosphino)ethyl]phosphine) were prepared by interaction of aqueous solutions of MCl42- salts with PP3 in CHCl3. Complexes 1 and 2 undergo facile chloro substitution reactions with KCN in 1:1 and 1:2 ratios to afford complexes [M(PP3)(CN)]CI [M = Pt (3)] and [M(PP3)(CN)](CN) [M = Pd (4), Pt (5)] possessing M-C bonds, both in solution and in the solid state. The reaction of 1 and 2 with SnCl2 in CDCl3 occurs with insertion of SnCl2 into M-Cl bonds leading to the formation of [M(PP3)(SnCl3)](SnCl3) [M = Pd (6), M = Pt (7)]. The isolation as solids of complexes 6 and 7 by addition of SnCl2 to the precursors requires the presence of PPh3 which activates the cleavage of M-Cl bonds, favors the SnCl2 insertion, and does not coordinate to M in any observable extent. Solutions of 6 in CDCl3 undergo tin dichloride elimination in higher proportion than solutions of 7. The reaction of complexes 1 and 2 with SnPh2Cl2 leads to [M(PP3)Cl](2)[SnPh2Cl4] [M = Pd (8)]. Complexes 2, 5, 7, and 8 were shown by X-ray diffraction to contain distorted trigonal bipyramidal monocations [M(PP3)X](+) [M = Pt, X = Cl- (2), X = CN- (5), X = SnCl3- (7); M = Pd, X = Cl- (8)], the central P atom of PP3 being trans to X in axial position and the terminal P donors in the equatorial plane of the bipyramids. The "preformed" catalyst 7 showed a relatively high aldehyde selectivity compared to most of the platinum catalysts.