Infrared Spectra of M(OH)₁,₂,₃ (M = Mn, Fe, Co, Ni) Molecules in Solid Argon and the Character of First Row Transition Metal Hydroxide Bonding

Reactions of laser-ablated Mn, Fe, Co, and Ni atoms with H(2)O(2) and with H(2) + O(2) mixtures in excess argon give new absorptions in the O-H and M-O stretching regions, which are assigned to metal dihydroxide and trihydroxide molecules, M(OH)(2) and M(OH)(3). Isotopic substitutions (D(2)O(2), (18)O(2), (16,18)O(2), D(2)) confirmed the assignments and DFT calculations reproduced the experimental results. The O-H stretching frequencies decreased in the dihydroxides from Sc to Zn. Mulliken and natural charge distributions indicate significant electron transfer from metal d orbitals to OH ligands that decreases from Sc to Zn, suggesting that the early transition metal hydroxides are more ionic and that the later transition metal hydroxides are more covalent.