MOLECULAR-STRUCTURE OF CRO2(NO3)2 IN THE GAS-PHASE - A NOVEL FORM OF COORDINATION FOR CHROMIUM

The gas-phase molecular structure of chromyl nitrate, CrO2(NO3)2, has been studied by electron diffraction at a temperature of 50-degrees-C and by ab initio methods at the HF level. The diffraction data are consistent with C2 symmetry for the molecule. The coordination about Cr is best described as derived from a severely distorted octahedron, since the nitrate groups act as bidentate ligands which are asymmetrically bonded to Cr. Chromium forms two multiple bonds to O atoms (Cr = O), two nominal single bonds to O(N) atoms (Cr-O), and two very long, presumably weak (estimated bond order 0.19-0.29) bonds (Cr <-- O) to other O(N) atoms on each of the nitrate groups. Given the doubt about the existence of CrF6, chromyl nitrate may be the only Cr(VI) species known to be six-coordinate. Principal distances (r(g)/angstrom) and angles (angle/deg), followed by estimated 2-sigma uncertainties in parentheses, are r(Cr = O) = 1.586 (2), r(Cr - O) = 1.957 (5), r(Cr <-- O) = 2.254 (20), r(N - O) = 1.341 (4), r(N = O(Cr)) = 1.254 (4), r(N = O) = 1.193 (4), angle(O = Cr = O) = 112.6 (35), angle(O - Cr - O) = 140.4 (33), angle(O = Cr - O4) = 97.2 (18), angle(O = Cr - O8) = 104.5 (9), angle(O = Cr <-- O) = 83.7 (34), angle(Cr - O - N) = 97.5 (5), angle(O - N = O(Cr)) = 112.2 (1), angle(O - N = O) = 119.7 (40), and angle(O = N = O) = 128.1 (36). The Cr-O-NO2 group is only slightly nonplanar with the dihedral angle of the planes Cr-O-N and NO2 equal to 16 (3)-degrees. It appears that there is relatively little torsional motion around either of the single bonds in the -O-NO2 groups. Results of geometry optimization at the SCF level with a large all-electron basis agree reasonably well with those from the electron-diffraction experiment and strongly support the conclusions about the nature of the coordination of the Cr atom. Tests with a pseudopotential for Cr, however, led to the unreasonable value 1.40 angstrom for r(C = O).