Effect of Coordination Geometry on the Gas-Phase Reactivity of Four-Coordinate Divalent Metal Ion Complexes

The gas-phase reactions of ammonia with M(EN-(py)2)2+, M(en)(phen)2+, and M(phen)22+, where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(II), or Zn(II), EN-(py)2 is 1,6-bis(2-pyridyl)-2,5-triazahexane, en is ethylenediamine, and phen is 1,10-phenanthroline, have been studied in a quadrupole ion trap mass spectrometer. The trends in reactivity as a function of the metal are noticeably different for each complex despite the similarity of the ligand donor groups. These results indicate that coordination geometry and electronic structure have a significant impact on the gas-phase reactivity of four-coordinate metal complex ions. Specifically, for M(EN-(py)2)2+ the equilibrium constants for the reactions with ammonia follow the trend Mn ≈ Fe ≈ Co > Zn ≫ Cu > Ni. The trend for the equilibrium constants of the M(en)(phen)2+ complexes is Mn ≈ Co ≫ Ni > Cu > Zn, and for the M(phen)22+ complexes, the trend is Ni > Mn ≈ Fe > Co > Cu > Zn. The most notable changes in reaction equilibrium constants occur for the Ni and Zn complexes, which increase and decrease, respectively, as the ligands are changed from EN-(py)2 to (en)(phen) to (phen)2. Molecular orbital stabilization energy (MOSE) and density functional theory (DFT) calculations are used to explain the experimental trends. Calculations indicate that the changes in reactivity observed for the Ni complexes are a result of different complex geometries; Ni(EN-(py)2)2+ is a low-spin square-planar complex, and Ni(phen)22+ is a high-spin tetrahedral complex. The decreased reactivity of the Zn complexes is due to the formation of a less distorted tetrahedral complex upon going from Zn(EN-(py)2)2+ to Zn(phen)22+. In addition, calculations show that the reactivities of the Mn, Fe, and Co complexes are consistent with slightly distorted tetrahedral structures, and the reactivities of the Cu complexes indicate that the complexes of this metal are close to square planar.