Molecular-orbital calculations have been performed for the conjugate base cis-[Co(NH3)4(NH2)Cl]+ and trans-[M(NH3)4(NH2)Cl]+ (M = CrIII, CoIII, and RhIII), the hexacoordinated intermediates cis- and trans-[Co(NH3)4(NH2)Cl]+, the square-pyramidal and trigonal bipyramidal pentacoordinated intermediates apical-[Co(NH3)4(NH2)]2+, basal-[Co(NH3)4(NH2)]2+, and equatorial-[M(NH3)4(NH2)]2+ (M = Co and Rh), respectively, using modified extended Hückel and SCF MS-X methods. The LUMO of the conjugate bases is an antibonding metal centered d* orbital which is stabilized during the dissociative activation of the MCl bond. For the above conjugate bases, separations of the highest doubly occupied MO (HDOMO) and the LUMO of 1.3, 2.5, 1.8, and 2.5 eV, respectively, have been calculated using the SCF MS-X model. Only in the conjugate base cis-[Co(NH3)4(NH2)Cl]+ with the smallest HDOMO - LUMO gap, the singlet electronic structure of the ground state may be stabilized by changing into a triplet when the CoCl bond is activated. This situation is unique to (acidato)(pentaamine)cobalt(III) complexes with deprotonated amine ligand cis to the leaving group and the reason for the existence of intermediates and their reactivity as well. Base hydrolysis of the analogues CrIII and RhIII complexes - the latter taken to represent the second- and third-row transition-metal amines - is unlikely to proceed via intermediates; a concerted substitution process is expected to take place.