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

Abstract

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) A, 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) A, Mo-Cl 2.38(2) A, and Mo-OH2 2.30(2) A. 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) A, Mo-(mu-O) 1.93(2) A, Mo-Cl 2.47(3) A, Mo-Mo 2.56(2) A, and a short Mo-OH2 distance of 2.15(2) A, 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-(mu-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<or=4.5 M, a conclusion supported by the Mo L2,3-edge XANES spectra.

From Yellow to Black: Dramatic Changes between Cerium(IV) and Plutonium(IV) Molybdates

“…[65][66][67][68][69][70][71][72][73] However, the energy at the 1/2-height of the edge is better, and indicates something close to Mo(VI), since Mo(IV) ½-height energies are typically below 20011 eV when using our chosen energy calibration. 63,64 With such uncertainty, it is not really possible to determine how much Mo(V) is mixed in; we therefore report here that the Mo K-edge XANES are consistent with a Mo(V,VI) state.…”

The Triple Salt 2(C7H9N4O2)[MoOCl4(H2O)] ⋅ 2(C7H9N4O2)Cl ⋅ (H17O8)Cl Containing a C2‐symmetrical Unbranched H+(H2O)8Zundel type Species in a Framework Composed of Theophyllinium, Aquatetrachloridooxidomolybdate and Chloride Ions

“…When dissolved in hydrochloric acid molybdenum(V) chloride reacts to form different complex anions, mostly depending on the pH value and the chloride ion concentration [13] given. Investigations using solutions with c (MoCl 5 )=0.2 mol ⋅ l −1 exemplarily showed that higher concentrations of hydrochloric acid ( c (HCl) > 6.3 mol l −1 ) lead to [MoOCl 5 ] 2− or [MoOCl 4 (H 2 O)] − anions as predominating species, while dilute hydrochloric acid solutions (4.5 mol ⋅ l −1 > ( c (HCl) > 1.7 mol ⋅ l −1 ) as a result of continuing steps of hydrolysis and olation mainly contain [Mo 2 O 4 Cl 4 (H 2 O) 2 ] 2− and [Mo 2 O 4 Cl 3 (H 2 O) 3 ] − ions [13] . Anyway, the hydrochloric acid solutions of molybdenum(V) chloride always contain several different complex anions, which are in a temperature, pH and concentration dependent equilibrium.…”

Direct observation of fractional change of niobium ionic species in a solution by means of X‐ray absorption fine structure spectroscopy

“…X‐ray absorption fine structure spectrum of transition metal around the near edge region is a promising tool to probe unoccupied d states. The pre‐edge peak intensity in the K‐edge X‐ray absorption near edge structure (XANES) of 3d and 4d transition metals is a good measure of coordination symmetry, and the L 2,3 ‐edge XANES spectrum reflects the local coordination and the valence of the X‐ray absorbing atoms . An L 3 ‐edge XANES spectrum from d 0 ion has two prominent peaks corresponding to the crystal field splitting of d orbitals, and the XAFS spectra were utilized to confirm the tetrahedral coordination of Mo(VI) in zeolites and to analyze the local structural changes of WO 3 species …”

“…The pre-edge peak intensity in the K-edge X-ray absorption near edge structure (XANES) of 3d and 4d transition metals is a good measure of coordination symmetry, [5,6] and the L 2,3 -edge XANES spectrum reflects the local coordination and the valence of the X-ray absorbing atoms. [7][8][9][10][11][12] An L 3 -edge XANES spectrum from d 0 ion has two prominent peaks corresponding to the crystal field splitting of d orbitals, and the XAFS spectra were utilized to confirm the tetrahedral coordination of Mo(VI) in zeolites [10] and to analyze the local structural changes of WO 3 species. [11] In the present paper, L 3 -edge XANES spectra were measured from niobium (V) oxalate solutions with different pH.…”