Origin of Polychromism of Cis Square-Planar Platinum(II) Complexes: Comparison of Two Forms of [Pt(2,2′-bpy) (Cl)₂]

Single crystals of the yellow form of [Pt(2,2′-bpy)(Cl)₂] have been prepared, and the crystal structure is compared to that of the Pd(II) homologue reported earlier. The yellow and red modifications of the title compound are compared by FTIR, UV/vis, EXAFS, and hydration studies. The major differences in the two forms arise from differences in the stacking mode of the monomers in the solid-state lattice, with significantly different Pt-Pt distances being observed in the yellow (4.435(1) Å) and red (3.45 Å) complexes. As expected from the similarity of the yellow forms of the Pt(II) complex to the corresponding Pd(II) complex, two inequivalent Pt-Cl distances are noted, in contrast to the architecture of the red form. In the latter, a pseudo-2-fold symmetry axis is preserved in the stacked array, while this symmetry element is absent in the yellow solid owing to a non-2-fold rotation around the Pt-Pt chain axis in the yellow form. The differences in the Pt-Pt scattering maxima extracted from the EXAFS results are in consonance with the crystallographic data.