Evidence for the dimer-of-(mixed-valent dimers) configuration in tetranuclear (μ4-TCNX)Ru(NH3)548+, TCNX = TCNE and TCNQ, from DFT calculations
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Monatshefte fuer Chemie 140 (7): 765--773 (2009)

DFT calcns. of the complex ions (μ4-TCNX)Ru(NH3)548+, TCNX = tetracyanoethene (TCNE) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), yield triplet state energy min. structures with nonplanar bridging ligands. The calcd. 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 Ru2.5Ru2.5 entities (Class III), agrees with exptl. results; the near-orthogonality found for the ground states of the mol. ions explains the obsd. 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 oxidn. The nitrile stretching frequency shifts were reasonably reproduced by the calcns. A d.-of-states representation for the TCNQ complex shows a rather different electronic structure in comparison to that for the formally related (μ4-TCNQ)Re(CO)3(bpy)44+, in particular a different frontier orbital situation. In contrast to the (TCNQ0)(ReI)4 situation, the tetraruthenium species with an approx. (TCNX2-)(Ru2.5)4 formulation represent unconventional mixed-valent tetranuclear compds.; in other words, weakly coupled pairs of strongly coupled dinuclear moieties. EPR spectroscopy at the W band frequency (95 GHz) of the TCNE compd. confirms that the redn. of the complex leads to the oxidn. of TCNE2- to yield a (TCNE·-)(RuII)4 species. on SciFinder(R)
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