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The reactions between [Mo.sub.2][([O.sub.2]C[super t]Bu).sub.4] and each of terephthalic acid and 4,4'-azodibenzoic acid have produced the compounds [[[Mo.sub.2][([O.sub.2]C[super t]Bu).sub.3]].sub.2]([mu]-[O.sub.2]C[C.sub.6][H.sub.4]C[O.sub.2]) and [[[Mo.sub.2][([O.sub.2]C[super t]Bu).sub.3]].sub.2]([mu]-[O.sub.2]C[C.sub.6][H.sub.4][N.sub.2][C.sub.6][H.sub.4]C[O.sub.2]), which are characterized by spectroscopic and electrochemical methods. The results have shown that the terephthalate bridge is more effective in electronic coupling between [Mo.sub.2] quadruply bonded centers than is the longer 4,4'-azodibenzoate.

The existence and persistence of quintuple bonding in isomers of model RMMR molecules of different geometry are investigated, where R = H, F, Cl, Br, CN, and C[H.sub.3] and M = Cr, Mo, and W. A low-symmetry [C.sub.s] structure is identified and the MM quintuple bond is persisting, regardless of the preference of the isomer of the RMMR molecule.

The electronic structures of D(sub 4h(super -))M(sub 2)(O2CH)(sub 4) and the oxalate-bridged complexes D(sub 2h(super -))[(HCO2)(sub 3)M2](sub 2)(mu-O2CCO2) and D(sub 4h(super -))[(HCO2)(sub 2)M2](sub 4)(mu-O2CCO2)(sub 4) are investigated by electronic structure calculations on the model formate complexes (M = Mo, W, Tc, Ru, and Rh). The study of oxalate shows that the greatest electronic coupling between M2 units will be for molybdenum and tungsten where the filled metal delta orbitals are of highest energy.

From the reactions between Mo2(O2CtBu)4 and each of terephthalic acid and 4,4′-azodibenzoic acid, the compounds [Mo2(O2CtBu)3]2(μ-O2CC6H4CO2) (1) and [Mo2(O2CtBu)3]2(μ-O2CC6H4N2C6H4CO2) (2) have been made and characterized by spectroscopic and electrochemical methods. Their electronic structures have been examined by computations employing density functional theory on model compounds where HCO2 substitutes for tBuCO2. On the basis of these studies, the two Mo2 units are shown to be only weakly coupled and the mixed-valence ions 1+ and 2+ to be valence-trapped and Class II and I, respectively, on the Robin-Day classification scheme for mixed- valence compounds. These results are compared to t2g6-Ru centers linked by 1,4-dicyanamidobenzene and azo-4,4′-diphenylcyanamido bridges for which the mixed-valence ions [Ru-bridge-Ru]5+ have been previously classified as fully delocalized, Class Ill [Crutchley et al. Inorg. Chem. 2001, 40, 1189; Inorg. Chem. 2004, 43, 1770], and on the basis of results described herein, it is proposed that the latter complex ion is more likely a mixed-valence organic radical where the bridge is oxidized and not the Ru(2+) centers. [ABSTRACT FROM AUTHOR]

With the aid of density function theory, the molecular and electronic structures of the molecules Mo2(O2CMe)4, MoW(O2CMe)4, and W2(O2CMe)4 and their single-electron oxidized radical cations have been determined; this includes calculated observables such as v(MM) and the δ → δ* electronic transition energies. The calculated properties are compared with those for the corresponding pivalates, M2(O2CtBu)4 (M = Mo or W) and MoW(O2CtBu)4 and their radical cations prepared in situ by oxidation with Cp2FePF6. The EPR spectra of the radical cations are also reported. The EPR spectrum of the MoW(O2CtBu)4+ cation reveals that the unpaired electron is in a polarized MM δ orbital having 70% Mo and 30% W character. The MM stretching frequencies show good correlation with the MM bond lengths obtained from single-crystal X-ray diffraction studies of MoW(O2CtBu)4, W2(O2CtBu)4, and W2(O2CtBu)4+PF6- compounds, along with previously reported structures. These data provide benchmark parameters for valence trapped dicarboxylate bridged radical cations of the type [(tBuCO2)3M2]2(μ-O2C-X-C02)+ (X = conjugated spacer). [ABSTRACT FROM AUTHOR]

The formation of [(η5-Ind)Ru(CO)2H] from [(η5-Ind)Ru(CO)3]+ was investigated using both 1H and 2H NMR spectroscopy and GC−mass spectrometry. Both NMR and GC-MS data support a water-gas shift reaction mechanism as the cause for formation of [(η5-Ind)Ru(CO)2H]. The formation of [(η5-Ind)Ru(CO)2H] from [(η5-Ind)Ru(CO)3]+ was investigated using both 1H and 2H NMR spectroscopy and GC−mass spectrometry. Both NMR and GC-MS data support a water-gas shift reaction mechanism as the cause for formation of [(η5-Ind)Ru(CO)2H]. [ABSTRACT FROM AUTHOR]

The effective conjugation of ortho and ortho-alt-para-arylene ethynylenes, with appropriately positioned pyridine and pyrazine heterocycles, increases upon binding to Ag(I) and Pd(II) cations. Significant bathochromic shifts in the electronic spectra, witnessed upon introduction of these metal bridges, are consistent with enhanced electron delocalization in the unsaturated backbone. Control studies suggest that this electronic behavior is attributable exclusively (in the case of Ag(I)) or partially (in the case of Pd(II)) to conformational restrictions of the conjugated backbones.

From the reactions between Mo2(O2CtBu)4 and each of terephthalic acid and 4,4'-azodibenzoic acid, the compounds [Mo2(O2CtBu)3]2(mu-O2CC6H4CO2) (1) and [Mo2(O2CtBu)3]2(mu-O2CC6H4N2C6H4CO2) (2) have been made and characterized by spectroscopic and electrochemical methods. Their electronic structures have been examined by computations employing density functional theory on model compounds where HCO2 substitutes for tBuCO2. On the basis of these studies, the two Mo2 units are shown to be only weakly coupled and the mixed-valence ions 1+ and 2+ to be valence-trapped and Class II and I, respectively, on the Robin-Day classification scheme for mixed-valence compounds. These results are compared to t2g6-Ru centers linked by 1,4-dicyanamidobenzene and azo-4,4'-diphenylcyanamido bridges for which the mixed-valence ions [Ru-bridge-Ru]5+ have been previously classified as fully delocalized, Class III [Crutchley et al. Inorg. Chem. 2001, 40, 1189; Inorg. Chem. 2004, 43, 1770], and on the basis of results described herein, it is proposed that the latter complex ion is more likely a mixed-valence organic radical where the bridge is oxidized and not the Ru(2+) centers.

With the aid of density function theory, the molecular and electronic structures of the molecules Mo2(O2CMe)4, MoW(O2CMe)4, and W2(O2CMe)4 and their single-electron oxidized radical cations have been determined; this includes calculated observables such as v(MM) and the delta --> delta* electronic transition energies. The calculated properties are compared with those for the corresponding pivalates, M2(O2CtBu)4 (M = Mo or W) and MoW(O2CtBu)4 and their radical cations prepared in situ by oxidation with Cp2FePF6. The EPR spectra of the radical cations are also reported. The EPR spectrum of the MoW(O2CtBu)4+ cation reveals that the unpaired electron is in a polarized MM delta orbital having 70% Mo and 30% W character. The MM stretching frequencies show good correlation with the MM bond lengths obtained from single-crystal X-ray diffraction studies of MoW(O2CtBu)4, W2(O2CtBu)4, and W2(O2CtBu)4+PF6- compounds, along with previously reported structures. These data provide benchmark parameters for valence trapped dicarboxylate bridged radical cations of the type [(tBuCO2)3M2]2(micro-O2C-X-CO2)+ (X = conjugated spacer).