STEREOELECTRONIC EFFECTS AND GAS-PHASE CO+, NI+, CPFE+, CPCO+ AND CPNI+ AFFINITIES OF PYRIDINES STUDIED BY THE KINETIC METHOD

Cluster ions of Co+, Ni+, CpFe+, CpCo+ and CpNi+ (Cp = cyclopentadieny l) with pyridines are generated in a multi-quadrupole mass spectromete r and the kinetic method is used to examine the pyridine affinities of the cations. Absolute Co+ and Nif affinities of pyridine are both det ermined 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 a ffinities of other ligands, such as water and ammonia, to these cation s. The relative metal ion and ligated-metal ion affinities of meta- an d para-substituted pyridines display excellent linear correlations wit h the corresponding proton affinities. The slopes of the correlation l ines for Co+, Ni+, CpFe+, CpCo+ and CpNi+ are 0.35, 0.41, 0.52, 0.42 a nd 0.49 respectively, the larger slopes being for the ligated metal io ns corresponding to their larger sizes and probable weaker bonding. A set of gas-phase stereoelectronic parameters S-k, measured by the devi ation of the experimental data from the metal ion/proton affinity regr ession line, shows that steric effects due to pyridine ring substituen ts give rise to lower than expected affinities for the di-ortho-substi tuted pyridines. Steric effects also weaken the pyridine-metal bonds i n the cyclopentadienyl metal ion-pyridine clusters compared to the bar e 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. (C) 1997 Elsevier Science S.A.