Five-coordinate [Pt(II)(bipyridine)2(phosphine)](n+) complexes: long-lived intermediates in ligand substitution reactions of [Pt(bipyridine)2](2+) with phosphine ligands.

The reaction of Pt(N-N)2 [N-N = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (4,4'-Me2bpy)] with phosphine ligands [PPh3 or PPh(PhSO3)2(2-)] in aqueous or methanolic solutions was studied by multinuclear ((1)H, (13)C, (31)P, and (195)Pt) NMR spectroscopy, X-ray crystallography, UV-visible spectroscopy, and high-resolution mass spectrometry. NMR spectra of solutions containing equimolar amounts of Pt(N-N)2 and phosphine ligand give evidence for rapid formation of long-lived, 5-coordinate Pt(II)(N-N)2(phosphine) complexes. In the presence of excess phosphine ligand, these intermediates undergo much slower entry of a second phosphine ligand and loss of a bpy ligand to give Pt(II)(N-N)(phosphine)2 as the final product. The coordination of a phosphine ligand to the Pt(II) ion in the intermediate Pt(N-N)2(phosphine) complexes is supported by the observation of (31)P-(195)Pt coupling in the (31)P NMR spectra. The 5-coordinate nature of [Pt(bpy)2{PPh(PhSO3)2}] is confirmed by X-ray crystallography. X-ray crystal structural analysis shows that the Pt(II) ion in [Pt(bpy)2{PPh(PhSO3)2}]·5.5H2O displays a distorted square pyramidal geometry, with one bpy ligand bound asymmetrically. These results provide strong support for the widely accepted associative ligand substitution mechanism for square planar Pt(II) complexes. X-ray structural characterization of the distorted square planar complex Pt(bpy)(PPh3)22 confirms this as the final product of the reaction of Pt(bpy)2 with PPh3 in CD3OD. The results of density functional calculations on Pt(bpy)2, Pt(bpy)2(phosphine), and Pt(bpy)(phosphine)2 indicate that the bonding energy follows the trend of Pt(bpy)(phosphine)2 > Pt(bpy)2(phosphine) > Pt(bpy)2 for stability and that the formation reactions of Pt(bpy)2(phosphine) from Pt(bpy)2 and Pt(bpy)(phosphine)2 from Pt(bpy)2(phosphine) are energetically favorable. These calculations suggest that the driving force for the formation of Pt(bpy)(phosphine)2 from Pt(bpy)2 is the formation of a more energetically favorable product.