Rate and equilibrium measurements for the hydrolysis of the Fe(CO)(3)-coordinated cyclohexadienyl cation lead to pK(R) = 4.7 compared with pK(R) = -2.1 for the uncoordinated ion. The hydrolysis yields exo-coordinated cyclohexadienol 10(7)-fold more rapidly than its endo-isomer, despite the isomers being of similar stability. DFT calculations of the energy of isodesmic transfer of Fe(CO)(3) from cyclohexadiene to benzene lead to an estimate of pK(a) similar to 8 for loss of a proton from the coordinated cyclohexadienyl cation to form Fe(CO)(3)-coordinated benzene. This implies that the coordinated cation is similar to 33 log units (46 kcal) less acidic than the uncoordinated ion.