Gas-phase reactions of divalent Ni complex ions with acetonitrile: Chelate ring size,inductive, and steric effects

Ni(II) complexes of a series of pentadentate polyamine ligands have been reacted with CH₃CN in the gas phase using a modified quadrupole ion trap mass spectrometer. The ligands have structural features such that upon complexation, chelate ring size, sterics, and inductive effects can be evaluated in the gas phase. Rate and equilibrium constants for CH₃CN addition to the metal complexes show that there is a general decrease in the gas-phase reactivity as the chelate ring size is increased. Density functional theory calculations at the B3LYP/LANL2DZ level of theory have been used to obtain minimum energy structures and Mulliken charges for the complexes. The decreased reactivity observed as the chelate ring size is increased correlates with a decrease in the atomic charge on the metal. A larger chelate ring size enhances ligand flexibility and improves the overlap of the ligand’s donor atoms with the metal center. Adding methyl groups adjacent to or on the nitrogen donor groups of a ligand also decreases the rate and equilibrium constants for the reactions of a given complex with CH₃CN. Analysis of Mulliken charges for these complexes indicates that both inductive and steric effects are responsible for lower complex reactivity. These results suggest that while the gas-phase reactivity of a metal complex with CH₃CN is very dependent on the functional groups directly bound to the metal, in some cases steric effects can conceal the correlation between reactivity and coordination structure.