By how much is protonated benzene stabilized by coordination of iron tricarbonyl?

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 10⁷-fold more rapidly than its endoisomer, 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 ∼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 ∼33 log units (46 kcal) less acidic than the uncoordinated ion.