The reactions of the square-planar methylplatinum-cis-diphosphine complexes, Pt(Me)(Cl){(S,S)-BDPP}, 1, and Pt(Me)(SnCl3){(S,S)-BDPP}, 2, (BDPP = (2S,4S)-2,4-bis(diphenylphosphino)pentane) with CO, and a mixture of CO/H-2 = 1/1 have been studied in CD2Cl2 solutions by variable temperature high-pressure NMR spectroscopy to establish the role of Pt-SnCl3 bond in the CO insertion and hydrogenolysis steps of the Pt-diphosphine-catalyzed olefin hydroformylation reaction. At low temperatures (193-213 K), the formation of the unprecedented four-coordinate ionic cis-methyl(carbonyl)platinum-diphosphine compound, [Pt(Me)(CO){(S,S)-BDPP}](+)X(-), where X = Cl, 3a, and X = SnCl3, 3b, has been observed whether CO is applied neat or in a mixture with hydrogen. The formation of compounds 3a and 3b involve equilibrium reactions which are facilitated by increasing the CO pressure; however, while the formation of 3b is a fast (magnitudes faster than that of 3a at identical conditions) and quantitative reaction, the formation of compound 3a remains a slow reaction that does not reach completion in the range of 1-50 bar of CO pressure. CO insertion in compounds 3a and 3b takes place only at temperatures close to ambient, yielding covalent acetyl compound Pt(COMe)(Cl){(S,S)-BDPP}, 4, and ionic acetyl compound [Pt(COMe)(CO){(S,S)-BDPP}](SnCl3-)-Sn-+, 5, respectively. At room temperature, 5 but not 4 reacts further in the presence of hydrogen, resulting in the formation of acetaldehyde and an unidendified Pt-diphosphine compound. Thus, it appears that the hydroformylation by Pt-BDPP complexes proceeds via an ionic reaction mechanism.