Polarization Effects of the Heavier Group 14 Elements on Vinylic Carbon–Carbon Double Bonds: Enabling an Unusual π-Coordination of Copper(I) Iodide in Coordination Polymers

The formation of CuI-vinyl π-complexes relies on a cooperative interplay between ligand polarization and donor softness. To investigate this concept experimentally, a series of vinylic tris(thioether) ligands CH₂═CH-E(CH₂SPh)₃ (E = <b>C</b>, <b>Si</b>, <b>Ge</b>) was synthesized and characterized in solution by multinuclear NMR spectroscopy, with <b>L1-Si</b> additionally studied in the solid state by X-ray crystallography. While the purely carbon-based ligand does not form a CuI complex under comparable conditions, the silicon- and germanium-substituted derivatives yield the one-dimensional coordination polymers [Cu(μ₂-I)₂Cu(μ-<b>L1-E</b>)₂]_n, featuring Cu(μ₂-I)₂Cu rhomboids bridged by thioether donors and a side-on π-bound vinyl group occupying the fourth coordination site. High-resolution X-ray diffraction data for <b>L1-Si</b> and <b>CP-Si</b> enabled multipole refinement and topological analysis of the experimental electron density. This approach directly confirms the vinyl-Cu(I) π-interaction and reveals subtle, polarization-dependent changes in bond topology induced by main-group substitution. Comparative studies on the isostructural germanium polymer indicate weaker π-bonding but uncover an unprecedented, reversible temperature-dependent site exchange of the Ge atom accompanied by thioether inversion. Overall, the electron-density analysis establishes main-group element polarization as a decisive factor in enabling vinyl-Cu(I) π-bonding and provides a robust experimental framework for the rational design of multifunctional Cu(I) coordination materials.