Diamagnetic Tp(i-Pr,Me)Co(CO)(2) (Tp(i-Pr,Me) = hydrotris(3-iso-propyl-5-methylpyrazolyl)borato) have been prepared and structurally characterized by X-ray diffraction. In solution it spontaneously loses carbon monoxide to yield paramagnetic Tp(i-Pr,Me)Co(CO), a complex with a triplet spin ground state. In the presence of CO, the two complexes are in equilibrium (i.e. Tp(i-Pr,Me)Co(CO) + CO double left right arrow Tp(i-Pr,Me)Co(CO)(2)), and measurements of the temperature dependence of the equilibrium, constant by variable temperature H-1 NMR spectroscopy have been used to determine the thermochemical parameters Delta H = -12.9(2) kcal and Delta S = -32(1) eu. The CO exchange reaction is fast on the NMR time scale at ambient temperature, but it can be frozen out below 224 K, the temperature of coalescence. At 224 K the rate constants for the dissociation and binding of CO were k(-1) = 6(1) x 10(3) s(-1) and k(1) = 3(1) x 10(9) M(-1) s(-1), respectively. This reaction represents an interconversion between a triplet and a singlet molecule by a typical organometallic two-electron process, and it is exceedingly fast. Based on this observation it is argued that the notion of "spin-blocking" of organometallic reactions is inappropriate.