The kinetics of the reactions of trialkylamine boranes and pyridine bo
ranes with benzhydryl cations have been determined photometrically. Se
cond-order rate laws are obeyed, first-order with respect ro amine bor
ane concentration and first-order with respect to carbocation concentr
ation. As for other reactions of carbocations with neutral nucleophile
s, the rates of these reactions are only slightly affected by solvent
polarity. The structure-reactivity relationships and kinetic isotope e
ffects are in accord with a polar mechanism proceeding through a trans
ition state where the migrating hydride is partly bound to the enterin
g carbon and to the leaving boron atom. The rate constants correlate l
inearly with the electrophilicity parameters E of the carbenium ions.
It is therefore possible to use the linear free enthalpy relationship
logk(2) = s(E + N) for determining nucleophilicity parameters N for th
e amine boranes and to compare their hydride-donating abilities with t
hose of other non-charged hydride donors (silanes, germanes, stannanes
, and dihydropyridines).