together, they suggest that this species has a linear or nearly linear µbridge. However, the large 180isotope effect seen for ys(Mo-0-Mo) and the high intensity of the otherwise forbidden vas(Mo-0-Mo) seen at 805 cm"1 are unexpected for a linearly bridged M-O-M ion. Calculations on the distortional isomers 2a and 2b show that moving the bridging oxygen away from the symmetrical position is slightly stabilizing in both isomers.31 Perhaps such a distortion is also operative in [MoAClg]4" to account for these effects.A linearly bridged Mo2034+ unit is expected to be diamagnetic, and in fact, all known examples of Mo2034+ complexes, including 2a and 2b, are diamagnetic.14,17 While paramagnetic Mo(V) species are very likely present in 6 M HC1 solution, an earlier suggestion2,35 that a "paramagnetic dimer" is responsible for the strong electronic absorption band at ~450 nm appears to warrant reinterpretation. A recent X-ray absorption study indicated the coexistence of significant fractions of the dinuclear Mo2042+ and Mo2034+ and the mononuclear Mo03+ species in 5 to 6 M HC1 solutions.36 This fact alone makes XAFS an ambiguous technique for the extraction of structural information. No information on the determination of phase shifts or multiple scattering was presented. Since second-shell distances are difficult to determine accurately and no authentic Mo2034+ complexes were used as models for the XAFS analysis, the evaluation of a Mo-O-Mo angle of ~107°is open to question. Furthermore, this study does (35) Hare,
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