Psh. Wong et al., STEREOELECTRONIC EFFECTS AND GAS-PHASE CO+, NI+, CPFE+, CPCO+ AND CPNI+ AFFINITIES OF PYRIDINES STUDIED BY THE KINETIC METHOD, Journal of organometallic chemistry, 539(1-2), 1997, pp. 131-139
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.