Vive la Difference– Mixed-metal Complexation
As mentioned already, the way a ligand goes about binding several metal ions is inevitably a stepwise process, as the probability of all metal ions attaching to the ligand at once is so low that it can be ignored. This suggests that one may be able to not only see some intermediate species, as they could be inherently stable, but also that one should be able to intercept these species and insert a different metal ion into an empty cavity to produce, not aM2L species, but a MML species (Figure 2.23). Because MML carries different metal ions, it will differ in chemical and physical properties from a M2LorM2L species with the same metal ions in each site. We can carry that concept a little further and examine a slightly different ligand set (Figure 2.24). Here, each of the two ligand head units is different. This means that when a first metal is coordinated, it has two inequivalent ML options– it can bind to the all-N set, or else the mixed-O. N set. When the second and different metal enters and binds to the remaining set, this then leads to two different MML species, provided there is no high selectivity of a metal for a particular head unit.
Figure 2.23
How the process of sequential complexation of metal ions may be applied to permit coordination to two different metal ions.
Thus, with this unsymmetrical ligand there are two complexes possible with two different metal ions inserted in different compartments. This simple example is sufficient to illustrate the often-vast range of options that becomes available when complicated ligands bind to metal ions. However, not all of these options will be equally probable, as the thermodynamic stability of each of the various options will differ.
Figure 2.24
How the process of choice and selection during complexation of metal ions to an unsymmetrical ligand may lead to two differing MML forms.
