Stereoelectronic effects and gas phase Co+, Ni+, CpFe+, CpCo+ and CpNi+ affinities of pyridines studied by the kinetic method

Abstract

Abstract Cluster ions of Co + , Ni + , CpFe + , CpCo + and CpNi + (Cp = cyclopentadienyl) with pyridines are generated in a multi-quadrupole mass spectrometer and the kinetic method is used to examine the pyridine affinities of the cations. Absolute Co + and Ni + affinities of pyridine are both determined as 60 ± 4 kcal mol −1 , values which are greater than the Fe + affinity (49 ± 3 kcal mol −1 ), consistent with the order of the affinities of other ligands, such as water and ammonia, to these cations. The relative metal ion and ligated-metal ion affinities of meta- and para-substituted pyridines display excellent linear correlations with the corresponding proton affinities. The slopes of the correlation lines for Co + , Ni + , CpFe + , CpCo + and CpNi + are 0.35, 0.41, 0.52, 0.42 and 0.49 respectively, the larger slopes being for the ligated metal ions corresponding to their larger sizes and probable weaker bonding. A set of gas-phase stereoelectronic parameters S k , measured by the deviation of the experimental data from the metal ion/proton affinity regression line, shows that steric effects due to pyridine ring substituents give rise to lower than expected affinities for the di-ortho-substituted pyridines. Steric effects also weaken the pyridine-metal bonds in the cyclopentadienyl metal ion—pyridine clusters compared to the bare metal systems. The occupancy of 4s orbitals in Fe + causes electronic repulsions with the pyridines and makes the steric effects larger for Fe + than the other metals, in spite of the fact that the Fe + systems are more weakly bound.