The kinetics of the reactions of dicobalt-coordinated propargyl cations with pi-nucleophiles (e.g., allylsilanes) and hydride donors (e.g., trialkylsilanes) have been studied conductometrically and/or photometrically. The reactions follow second-order kinetics with rate-determining CC-bond formation or hydride transfer. It is found that phenyl and trimethylsilyl substituents in the propargyl cation moiety reduce the electrophilic reactivities of these cations by less than a factor of 10. The electrophilicity is reduced by a factor of more than 10(5), however, when one CO ligand is replaced by PPh3 (3a --> 3e). The reactions of the dicobalt-coordinated propargyl cations with allylsilanes and - stannanes, silylated enol ethers and ketene actetals, and hydride donors (R3SiH, R3SnH) follow the linear free enthalpy relationship log k = s(E + N), which allows one to calculate electrophilicity parameters E for these carbocations and to rationalize their synthetic potential.