Geometric and electronic structure of the aqueous Al(H₂O) ₆³⁺ complex

The bonding environment of the aqueous A1(H₂O)₆ ³⁺ complex was studied using X-ray absorption near-edge structure (XANES) spectroscopy at the Al K-edge, with spectral interpretations based on density functional theory (DFT). Calculations for a highly symmetric complex (T_h symmetry) indicate electron transitions into Al 3p - O 2s and Al 3 p - O 2p antibonding orbitals, with a split O 2p contribution that appears to be due to a weak π-interaction of the Al 3p orbitals with water ligands off-axis (equatorial) with respect to the Al 3p axis. Calculations were performed with several hypothetical structures to assess the effects of Al - O bond length, orientation of water ligands in the first coordination shell, and the presence of a second solvation shell on the XANES spectrum. Similar transitions were observed in all of these cases, but with further splitting on addition of 12 solvation waters, inward tilting and random twisting of the water ligands, and nonuniform Al - O bond lengths. Although it was previously hypothesized that the broadness of the XANES spectrum for this complex is due to an asymmetric geometry, these results illustrate how an Al(H₂O) ₆³⁺ geometry that is octahedral (Oh) with respect to the Al - O₆ core could produce the broad spectrum observed. Because geometric distortions would affect relative Al - O bond strengths, an understanding of the equilibrium Al(H₂O)₆³⁺ geometry is prerequisite to a quantitative description of reaction chemistry, including acidity and ligand exchange.