Effect of Metal-Ligand Bond Distances on Rates of Electron-Transfer Reactions. The Crystal Structures of Hexaammineruthenium(II) Iodide, [Ru(NH₃)₆]I₂, and Hexaammineruthenium(III) Tetrafluoroborate, [Ru(NH₃)₆][BF₄]₃

The crystal and molecular structures of hexaammineruthenium(II) iodide and hexaammineruthenium((II) tetrafluoroborate have been determined from three-dimensional X-ray data collected at room temperature by counter methods. The two complexes both crystallize with four formula units in the cubic space group Oh⁶-Fm3m, with m3m site symmetry imposed on the cations Ru^II(NH₃)₆²⁺ and Ru^II(NH₃)₆³⁺. The ruthenium(II) complex has a unit cell edge a = 10.841 (4) Å. The structure has been refined by least-squares techniques to a conventional R factor of 2.1% on F, using 139 unique reflections for which F² = 3σ(F²). The ruthenium(III) complex has a unit cell edge a = 11.385 (1) Å. The least-squares refinement on F, using 385 independent reflections for which F² = 3σ(F²), converged to a conventional R factor of 4.7%. The Ru(II)-NH₃ distance is 2.144 (4) Å and the Ru(III)-NH₃ distance is 2.104 (4) Å, a significant difference of 0.040 (6) Å. This result in conjunction with the known difference of 0.178 (17) A between the Co(II)-NH₃ and Co(III)-NH₃ bond distances leads to the conclusion that the electrostatic effect on bond distances is significantly smaller than the effect of spin change. From a calculation of ligand field stabilization energy it is found that the anomalous slowness of the electron transfer between Co(NH))₆²⁺ and Co(NH₃)₆³⁺ arises from the.difference in energy between the spin states of the two exchanging ions and not from the bond length reorganization energy.