Reactivity of hydrated monovalent first row transition metal ions [M(H₂O)_n]⁺, M = Cr, Mn, Fe, Co, Ni, Cu, and Zn, n <50, toward acetonitrile

Reactions of [M(H₂O)_n]⁺, M = Cr, Mn, Fe, Co, Ni, Cu, and Zn, n <50, with CH₃CN are studied in the gas phase by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Sequential uptake of 4-6 acetonitrile molecules is observed for all metals. Rate constants show a weak dependence on both the metal and the number of acetonitrile molecules already in the cluster. Nanocalorimetry yields the enthalpy of the first reaction step. For most metals, this is consistent with a ligand exchange of water against acetonitrile. For M = Cr, however, the strong exothermicity of ΔE_nc = -195 ± 26 kJ mol^-1 suggests an electron transfer from Cr⁺ to CH₃CN. Exclusively for M = Zn, a relatively slow oxidation of the metal center to Zn²⁺, with formation of ZnOH⁺ and release of CH₃CNH^• or CH₃CHN^• is observed. Density functional theory molecular dynamics simulations and geometry optimizations show that charge transfer from Zn⁺ to CH₃CN as well as the subsequent proton transfer are associated with a barrier.