Abstract
Infrared vibrational predissociation spectra of transition metal hydroxide clusters, [MOH](+)(H2O)1-4·D2 with M = Mn, Fe, Co, Ni, Cu, and Zn, are presented and analyzed with the aid of density functional theory calculations. For the [MnOH](+), [FeOH](+), [CoOH](+) and [ZnOH](+) species, we find that the first coordination shell contains three water molecules and the four ligands are arranged in a distorted tetrahedral geometry. [CuOH](+) can have either two or three water molecules in the first shell arranged in a planar arrangement, while [NiOH](+) has an octahedral ligand geometry with the first shell likely closed with five water molecules. Upon closure of the first coordination shell, characteristic stretch frequencies of hydrogen-bonded OH in the 2500-3500 cm(-1) region are used to pinpoint the location of the water molecule in the second shell. The relative energetics of different binding sites are found to be metal dependent, dictated by the first-shell coordination geometry and the charge transfer between the hydroxide and the metal center. Finally, the frequency of the hydroxide stretch is found to be sensitive to the vibrational Stark shift induced by the charged metal center, as observed previously for the smaller [MOH](+)(H2O) species. Increasing solvation modulates this frequency by reducing the extent of the charge transfer while elongating the M-OH bond.
Highlights
Interactions between a metal center and its ligands are important for the stability of the complex as well as its functionality
The extent of charge transfer between a metal center and a ligand group can be influenced by interactions with nearby polar solvents,[1,2,3,4] and the structural arrangement of the ligand groups can vary depending on local solvation environment
Structures of hydrated singly and doubly charged alkali and transition metal clusters have been very well characterized by photodissociation spectroscopy.[15,16,17,18,19,20,21,22,23,24,25,26]. Building on these results of solvated metal ions, we extend our study of the [MOH]+ species to larger clusters to probe the effect of solvation on the M–OH charge transfer as well as to determine how the coordination sphere is influenced by the charged hydroxide ligand
Summary
Interactions between a metal center and its ligands are important for the stability of the complex as well as its functionality. The observed opposing trends in the hydroxide stretch for [MgOH]+(H2O)1–5, [CaOH]+(H2O)[1,2,3,4,5], and [CuOH]+(H2O)[1,2,3] clusters are likely reflecting different solvent-driven changes in the metal–ligand charge transfer Another open question is how the presence of a charged ligand affects the solvation structure of the metal complex. Structures of hydrated singly and doubly charged alkali and transition metal clusters have been very well characterized by photodissociation spectroscopy.[15,16,17,18,19,20,21,22,23,24,25,26] Building on these results of solvated metal ions, we extend our study of the [MOH]+ species to larger clusters to probe the effect of solvation on the M–OH charge transfer as well as to determine how the coordination sphere is influenced by the charged hydroxide ligand. The results point to the water molecules acting as a polarizable medium that affects the charge-transfer interaction even in the molecular solvation regime
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