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The Cause for Tremendous Acceleration of Chloride Substitution via Base Catalysis in the Chloro Pentaammine Cobalt(III) Ion

The base hydrolysis reaction of Co(NH3)5Cl2+ was investigated using density functional theory and molecular orbital methods. Geometries and energies of conjugate bases, intermediates, transition states, and minimum energy crossing points were computed. For the base hydrolysis of Co(NH3)5Cl2+, three pathways might operate: the mechanism proposed by Basolo and Pearson, the mechanism via a hexacoordinated intermediate exhibiting a triplet ground state, and a fully stereomobile Id mechanism. The hexacoordinated intermediate can lose the leaving ligand readily to form a square pyramidal pentacoordinated intermediate with a triplet state, which interconverts rapidly and reversibly into the Basolo–Pearson trigonal bipyramid with a singlet state. Due to its high activation energy, a stereochemical rearrangement via a Berry pseudorotation does not take place. The intermediates are not protonated because their pKa values are 5 for the hexacoordinated intermediate and −6 for the trigonal bipyramidal pentacoordinated intermediate. The Basolo–Pearson mechanism proceeds with 50% stereoretention and 50% stereomobility. In the case that the hexacoordinated intermediate converts into the trigonal bipyramid, products with the same stereochemistry would be obtained. If, however, for the square pyramidal intermediate with a triplet state the entering ligand competes efficiently with the rearrangement into the trigonal bipyramid or if the substitution takes place at the hexacoordinated intermediate via, e.g., the Id mechanism, the reaction would proceed with retention of the configuration. Direct substitution via the Id mechanism, operating for azide and to a small extent also for water, is fully stereomobile. Computations on the Basolo–Pearson mechanism have also been performed for the chloro pentaammine complexes of chromium(III), ruthenium(III), and rhodium(III). This pathway might operate for chromium(III) but not for ruthenium(III) and rhodium(III).

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