Lability and Inertness in Octahedral Complexes
We have already introduced the concept of rate of change for complexes expressed in terms of the two extremes of labile (fast) and inert (slow), where charge and size are key factors. Experimental observations of reactivity allow us to define, at least approximately which metal ions fall into which category. For transition metals, although size/charge effects are important, they do not fully explain experimental observations. It was Henry Taube who showed that the d-electron configuration played an important role, and that high-spin complexes are generally labile. The following groupings were identified from experimental observations of octahedral complexes:
labile– all complexes where the metal ion has electrons in eg∗ orbitals [e.g. Co2+ (d7: t2g5eg2); high spin Fe3+ (d5: t2g3eg2)] and all complexes with less than three d electrons [e.g. Ti3+ (d1)];
inert– octahedral d3 complexes [e.g. Cr3+ (d3: t2g3eg0)], and low spin d4.d5 and d6complexes [e.g. Co3+ (d6: t2g6eg0)].
An analysis in terms of the relative energies of the precursor and reaction intermedi ate predicts lability for octahedral complexes with populated eg∗ levels in line with the experimental observations.
There are, of course ‘grey’ areas. Even within a traditionally inert system, reactivity may vary markedly with the type of ligand undergoing reaction. For [CoIII (NH3)5X] n+ reacting in water to replace the X-group by a water molecule, the rate constant varies by an order of∼1010 from NH3 (k = 5.8 × 10-12s-1 at room temperature, a half-life of 3800 years) to OClO3− (k = 1.0 × 10−2s−1. a half-life of 7 seconds). Such variations reflect the nature of the ligand more than the metal, since the ligand lability or capability as a good leaving group tends to extend across a wide range of metal ions.
