Solution studies of Ru-2(O(2)CR)(4)(n)+ complexes (n=0, 1; O(2)CR=octanoate, crotonate, dimethylacrylate, benzoate, p-toluate) and solid-state structures of Ru-2(O2C-p-tolyl)(4)(THF)(2), [Ru-2(O2C-p-tolyl)(4)(THF)(2)](+)[BF4](-), and Ru-2(O2C-p-tolyl)(4)(CH3CN)(2): Investigations of the axial ligation of the Ru-2 core

Ru-2(O2C(CH2)(6)CH3)(4) (1a) is soluble in both coordinating (THF, CH3OH, CH3CN) and noncoordinating solvents (benzene, toluene, cyclohexane, CH2Cl2), allowing its solution properties to be investigated by H-1 and C-13 NMR spectroscopy, UV/visible spectroscopy, resonance Raman spectroscopy, and cyclic voltammetry. In noncoordinating solvents, la exists as an oligomer, presumably by way of axial intermolecular -(--[Ru-2]--O--)(n)- interactions. H-1 NMR studies of 1a and [Ru-2(O2C(CH2)(6)CH3)(4)](+)[X](-)([1a](+)[X](-)) where X = Cl, BF4, or O2C(CH2)(6)CH3, indicate that both dipolar and contact mechanisms contribute to the paramagnetic shifts of the protons. Resonances for axial and equatorial ligands are shifted upfield and downfield, respectively, by a dipolar mechanism. Aromatic ligands in the axial sites, e.g. pyridine and pyrazine, experience an enhanced upfield shift by direct pi-delocalization. Comparison of the H-1 NMR signals for M(2)(O(2)CR)(4) compounds where M = Ru and O(2)CR = benzoate, toluate, butyrate, crotonate, and dimethylacrylate with those where M = Mo indicates that the equatorial carboxylate ligands in the diruthenium species also experience pi-contact shifts. Variable-temperature studies and calculated estimates of dipolar shifts (using structural parameters taken from solid-state structures) indicate a significant zero-field splitting contribution to the dipolar shift. The arrangements of the toluate rings in Ru-2(O2C-p-tolyl)(4)(THF)(2), Ru-2(O2C-p-tolyl)(4)(CH3CN)(2), and [Ru-2(O2C-p-tolyl)(4)(THF)(2)](+)[BF4](-) deviate by 15(1), 2.3(2), and 7.3 degrees, respectively, from alignment with the Ru-Ru axis. The Ru-Ru distances for the two neutral and the cationic complexes are 2.27(1) Angstrom, i.e. not significantly affected by the nature of the axial ligand (THF versus CH3CN) or by charge n+ (n = 0, 1). The cell parameters for Ru-2(O2C-p-tolyl)(4)(THF)(2) . 2THF at -154 degrees C are a = 10.730(5) Angstrom, b = 12.335(6) Angstrom, c = 9.193(4) Angstrom, alpha 105.15(2)degrees, beta = 109.35(2)degrees, gamma = 77.98(2)degrees, Z = 2 (asymmetric unit is RuC24H30O6), d(calcd) 1.559 g/cm(3), and space groug P (1) over bar. The cell parameters for Ru-2(O2C-p-tolyl)(4)(CH3CN)(2) . CH3CN at -169 degrees C are a = 27.058(3) Angstrom, b = 10.049(1) Angstrom, c = 17.956(2) Angstrom, beta = 120.89(1)degrees, Z = 4, d(calcd) = 1.465 g/cm(3), and space group C2/c. The cell parameters for [Ru-2(O2C-p-tolyl)(4)(THF)(2)](+)[BF4](-) at -172 degrees C are a = 13.056(4) Angstrom, b = 21.358(6) Angstrom, c = 9.199(2) Angstrom, beta = 111.28(1)degrees, Z = 2, d(calcd) = 1.350 g/cm(3), and space group C2/m.