The pK(a) values of a cationic selenyl- (5H(+)) and a benzothienylcarbene complex (6H(+)) and rate constants for the reversible deprotonation of these complexes by water, carboxylate ions, primary aliphatic amines, secondary alicyclic amines (5H(+) only), and OH- (5H(+) only) were determined in 50% MeCN-50% water (v/v) at 25 degreesC. In comparison with neutral Fischer-type carbene complexes such as 1H, the cationic complexes 5H(+) and 6H(+) are much more acidic, and the intrinsic barriers to proton transfer are substantially higher. This paper discusses a variety of factors that contribute to these differences, with the most important ones being that 5H(+) and 6H(+) are cationic, which makes the C5H5(NO)(PPh3)Re moiety a stronger pi -acceptor than the (CO)(5)M moieties, coupled with the fact that the deprotonated forms of 5H(+) and 6H(+) are aromatic molecules.