Bond dissociation energies ligand substitution process

Low-spin complexes in which the metal configuration is d3, d4, d5, or d6 would require either a ligand to leave before substitution could occur or the utilization of an outer d orbital to form a seven-bonded transition state. Such complexes frequently undergo substitution by a dissociative process because expanding the coordination number of the metal is made difficult by the lack of a vacant orbital of suitable energy. In either the SN1 or the SN2 case, the substitution should be much slower than it is for labile complexes. Accordingly, complexes of V2+, Cri+, Mn4+ (all of which are d3 ions), low-spin complexes of Co3+, Fe2+, Ru2+, Rh3+, Ir3+, Pd4+, and Pt4+ (all of which are dt ions) and low-spin complexes of Mn3+, Re3+, and Ru4+ are classified as inert. Inert does not mean that substitution does not occur, but rather that it occurs much more slowly than it does in labile complexes. 

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