Coordination Numbers of Hydrated Divalent Transition Metal Ions Investigated with IRPD Spectroscopy

Hydration of the divalent transition metal ions, Mn, Fe, Co, Ni, Cu, and Zn, with 5–8 water molecules attached was investigated using infrared photodissociation spectroscopy and photodissociation kinetics. At 215 K, spectral intensities in both the bonded-OH and free-OH stretch regions indicate that the average coordination number (CN) of Mn2+, Fe2+, Co2+, and Ni2+ is ∼6, and these CN values are greater than those of Cu2+ and Zn2+. Ni has the highest CN, with no evidence for any population of structures with a water molecule in a second solvation shell for the hexa-hydrate at temperatures up to 331 K. Mn2+, Fe2+, and Co2+ have similar CN at low temperature, but spectra of Mn2+(H2O)6 indicate a second population of structures with a water molecule in a second solvent shell, i.e., a CN < 6, that increases in abundance at higher temperature (305 K). The propensity for these ions to undergo charge separation reactions at small cluster size roughly correlates with the ordering of the hydrolysis constants of these ions in aqueous solution and is consistent with the ordering of average CN values established from the infrared spectra of these ions.