Evidence for the dimer-of-(mixed-valent dimers) configuration in tetranuclear {(mu(4)-TCNX)[Ru(NH3)(5)](4)}(8+), TCNX = TCNE and TCNQ, from DFT calculations

DFT calculations of the complex ions {(mu(4)-TCNX)[Ru(NH3)(5)](4)}(8+), TCNX = tetracyanoethene (TCNE) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), yield triplet state energy minimum structures with nonplanar bridging ligands. The calculated C-C bond distances and twist angles confirm considerable metal-to-ligand electron transfer from the metal centers to the TCNE and TCNQ bridges in the lowest triplet and singlet states. The resulting situation, involving a weak interaction between two strongly coupled malonodinitrilato-bridged (RuRu2.5)-Ru-2.5 entities (Class III), agrees with experimental results; the near-orthogonality found for the ground states of the molecular ions explains the observed magnetic exchange coupling between two S = 1/2 sites, the 1,258 cm(-1) absorption in the IR spectrum of the TCNE complex, and the reversible two-electron oxidation. The nitrile stretching frequency shifts were reasonably reproduced by the calculations. A density-of-states representation for the TCNQ complex shows a rather different electronic structure in comparison to that for the formally related {(mu(4)-TCNQ)[Re(CO)(3)(bpy)](4)}(4+), in particular a different frontier orbital situation. In contrast to the (TCNQ(0))(Re-I)(4) situation, the tetraruthenium species with an approximate (TCNX2-)(Ru-2.5)(4) formulation represent unconventional mixed-valent tetranuclear compounds; in other words, weakly coupled pairs of strongly coupled dinuclear moieties. EPR spectroscopy at the W band frequency (95 GHz) of the TCNE compound confirms that the reduction of the complex leads to the oxidation of TCNE2- to yield a (TCNE center dot-)(Ru-II)(4) species.