Synthesis, crystal structure, and spectroscopic properties of [Mo2(O2CMe)2(MeCN)6][BF4]2, [Rh2(O2CMe)2(MeCN)6][BF4]2, and [Rh2(O2CMe)2(MeCN)4(py)2][BF4]2

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Abstract
Treatment of [M2(O2CMe)4](M = Mo or Rh) with [OMe3][BF4](ca. 1 : 4 mol ratio) in MeCN produces the corresponding [M2(O2CMe)2(MeCN)6][BF4]2 compound; dissolution of the rhodium derivative in pyridine (py) produces [Rh2(O2CMe)2(MeCN)4(py)2][BF4]2. The structure of each of these compounds has been confirmed by X-ray crystallography: [Mo2(O2CMe)2(MeCN)6][BF4]2 crystallises in the monoclinic space group P21/m, with a= 7.160(1), b= 10.642(1), c= 19.402(2)Å, β= 99.47(1)°, and Z= 2; [Rh2(O2CMe)2(MeCN)6][BF4]2 crystallises in the monoclinic space group C2/c, with a= 15.749(2), b= 18.417(3), c= 13.965(2)Å, β= 91.34(2)°, and Z= 4; [Rh2(O2CMe)2(MeCN)4(py)2][BF4]2 crystallises in the monoclinic space group C2/m, with a= 19.448(3), b= 10.476(1), c= 18.990(3)Å, β= 113.25(1)°, and Z= 4. The final R factors (and observed reflections) for the refined structures were 0.053 (2 774), 0.040 (2 936), and 0.059 (2 391), respectively. In each compound, the cation involves a cis arrangement of the two µ-acetato-groups, four equatorial N-bonded MeCN molecules, and two axial ligands (MeCN or pyridine) co-ordinated to the dimetal(II) centre. For each of the cations, the bond lengths are normal for such species and comparisons within and between values for the [M2(O2CR)2(MeCN)6]2+(M = Mo or Rh) provide a clear indication of the static trans effect of a Mo[graphic omitted]Mo quadruple bond and a Rh–Rh single bond, together with a clear documentation of the structural similarities and differences for analogous Mo2 4+ and Rh2 4+ complexes: Mo–Mo 2.136(1), Mo–Oeq 2.084(13), Mo–Neq 2.148(4), Mo–Nax 2.759(19); Rh–Rh 2.534(1), Rh–Oeq 2.015(4), Rh–Neq 1.983(4), and Rh–Nax 2.232(4)Å. The dimensions of the Rh2O4N6 cores of [Rh2(O2CMe)2(MeCN)4L2]2+(L = MeCN or py) are virtually identical; the latter complex involves an orthogonal arrangement of the two pyridine rings. Proton and 13C n.m.r. studies have shown that, at ambient temperatures, both the axial and equatorial MeCN molecules of [Mo2(O2CMe)2(MeCN)6]2+ undergo rapid exchange with CD3CN, whereas for the corresponding rhodium complex only the axial molecules undergo such exchange. U.v.-visible and i.r. data are also reported.

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