On the elusive anti-bayerite structure

• Cr-4s/O-2s mixing controls sequential deprotonation of the Cr3+ hexahydrate.
• Hydrogen bonding was between ligands and second hydration shell molecules.
• Intramolecular hydrogen bonding was not detected.
• Active hydroxide was formed by a 2:1 to 3:1 ratio of facial to meridional isomers.
• The active hydroxide maintains six H-bonds with 2nd hydration shell molecules.

Sequential deprotonation of the Cr3 + hexahydrate in an alkaline environment up to the stage of a charge-neutral active hydroxide was studied via density functional theory. The deprotonation could be characterized as autocatalytic since upon completion of every H-abstraction stage, Cr was found to mediate O–H dissociation in the next stage by pre-conditioning the ligand O atom that contributes the highest 2s density into Cr-4s based molecular orbitals; the latter amounts to a greater Cr–O distance due to increased charge density along the Cr–O axis. A direct effect of such Cr-4s/O-2s mixing is the reduction of electronegativity of the ligand-O atom and a corresponding high Voronoi deformation density (VDD) of the attached ligand-H atoms. Based on bonding energy decomposition, a facial to meridional isomer ratio of between 2:1 and 3:1 was derived as the most probable stereochemical mix of the active hydroxide; the latter forms, by mutual donation and acceptance, six hydrogen bonds with second hydration shell molecules.