The pK(a) of the title compound, 3, was determined kinetically to be 14.47 in 50% MeCN-50% water at, 25 degrees C. Rate constants for the reversible deprotonation of 3 by OH- and piperidine, and primary kinetic isotope effects as well as kinetic solvent isotope effects for the deprotonation of 3 by OH- (OD-), are reported. Comparison of our results with corresponding data for methoxymethylcarbenepentacarbonylchromium (2a), shows that the pK(a) of 3 is about 2 units higher than that of 2a. This reduction in acidity is mainly attributed to enhanced pi-donation from the oxygen to the carbene carbon of 3, which results from the ring structure of 3. The intrinsic rate : constants for the deprotonation of 3 are essentially the same as for the deprotonation of 2a. This suggests that the expected intrinsic rate lowering effect due to the increased resonance stabilization of 3 is offset by an intrinsic rate enhancing preorganization effect.