Structural Characterization of Molybdenum(V) Species in Aqueous HCl Solutions

Mo(V) aqua-chloro complexes in hydrochloric acid solutions have been studied by means of Mo K- and L2,3-edge
X-ray absorption and Raman spectroscopic methods. The solid compounds (HPPh3)2[MoOCl5] (1), 6[MoOCl4(H2O)]-·10(pyH)+·4Cl- (2), and (pyH)2[Mo2O4Cl4(trans-OH2)2] (3) were used for structural comparisons. The compound 2
crystallizes in the orthorhombic space group Pmma (no. 51) with a = 21.398(3), b = 8.057(4), c = 13.330(4) Å,
and Z = 4. In 0.2 M solutions of MoCl5 in 7.4−9.4 M HCl the mononuclear [MoOCl4(OH2)]- complex dominates
with the bond distances MoO 1.66(2) Å, Mo−Cl 2.38(2) Å, and Mo−OH2 2.30(2) Å. Its Raman band at 994 cm-1
for the MoO symmetric stretching vibration is closer to that of 2 (988 cm-1) than of 1 (969 cm-1). The Mo
K-edge EXAFS spectrum for 0.2 M MoCl5 in 1.7 M HCl solution reveals a dinuclear [Mo2O4Cl6-n(OH2)n]n-4 (n =
2, 3) complex with a double oxygen bridge and the average distances MoO 1.67(2) Å, Mo−(μ-O) 1.93(2) Å,
Mo−Cl 2.47(3) Å, Mo−Mo 2.56(2) Å, and a short Mo−OH2 distance of 2.15(2) Å, which implies that at least one
of the aqua ligands is in equatorial position relative to the two axial MoO bonds. This position differs from the
Mo−OH2 configuration exclusively trans to the MO groups of the isomeric (with n = 2) dinuclear complex in 3.
The difference in the ligand field is also reflected in their L2,3-edge XANES spectra. For 0.2 M MoCl5 solutions in
intermediate HCl concentrations (3.7−6.3 M) the Raman bands at 802 cm-1 (Mo−O−Mo) and 738 cm-1 (Mo−(μ-O)2−Mo) verify three coexisting classes of Mo(V) complexes:  mononuclear complexes together with dinuclear
mono-oxo (e.g., [Mo2O3Cl6(H2O)2]2-) and dioxo bridged species, even though principal component analysis (PCA)
of the corresponding series of EXAFS spectra only could distinguish two major components. By fitting linear
combinations of the appropriate EXAFS oscillation components, dioxo-bridged dinuclear complexes were found to
dominate at HCl concentrations ≤ 4.5 M, a conclusion supported by the Mo L2,3-edge XANES spectra.