Kinetics Inert and Labile Complexes

Any complex formation takes place in a substitution reaction by replacement of one ligand by another. Thus, any substitution reaction is fundamentally a Lewis acid-base reaction.

The rate of a substitution reaction is primarily determined by the ratio between charge and size (charge density) of the metal ion, but when transition metals are involved the ¿-electron structure should also be taken into account. The term "labile" will be used for very reactive complexes while less reactive ones are called inert. Care should be taken not to confuse the term "labile" (kinetic) with the thermodynamic designation, stable.

Knowledge of the kinetic properties of complexes will obviously be decisive in the design of drugs. If a pharmaceutical in the form of an organic molecule is transferred to a target site by means of a metal ion, the complex should not be highly inert. On the other hand, complex formation between Pt(II) and DNA bases should be sufficiently inert in order to have adequate time to affect the division of tumor cells.

The electronic structure of transition metal ion complexes determines their reactivity due to the particular occupancy of the ¿-orbitals. Following the crystal field theory, the five ¿-orbitals split in the presence of the electrostatic field provided by the ligands (crystal field).

The doubly degenerate energy levels are denominated e while the triply degenerate levels are called t2. The two high-energy ¿-orbitals of an octahedral complex are thus type e-orbitals while the

FIGURE 10.3 Crystal splitting of rf-orbitals. The diagram shows the splitting of a set of rf-orbitals in a metal ion complex having an octahedral symmetry. The energy difference between e and t2 orbitals is designated A0.

FIGURE 10.3 Crystal splitting of rf-orbitals. The diagram shows the splitting of a set of rf-orbitals in a metal ion complex having an octahedral symmetry. The energy difference between e and t2 orbitals is designated A0.

e three lower-lying orbitals are of the t2 type (cf. Figure 10.3). The energy difference, A0, is called the ligand field splitting. In a tetrahedral coordination compound the arrangement is the opposite.

By preferentially filling up the lower-lying t2-orbitals the rf electrons will stabilize the system relative to an average arrangement of the electrons among all available orbitals. The gain in binding energy obtained by distributing the charges in a nonsymmetrical way is called crystal field stabilization energy (CFSE). The e-orbitals clearly have higher energy than the t2-orbitals. We now assign an energy of -2/5 x A0 to the three t2-orbitals and +3/5 x A0 to the e-orbitals, and can calculate the stabilization energies for complexes with any number of rf electrons. For example, a rf5 high-spin octahedral complex will acquire a CFSE of (-3 x 2/5 + 2 x 3/5) x A0 equal to 0. In a low-spin rf5 complex, the energy will be lowered by -5 x 2/5 x A0 or -2 x A0. Thus, the latter will be considerably less reactive than the former.

The rf8 configuration deserves special attention since this system leads to very stable and inert square planar compounds. Platinum(II) complexes belong to this group and will be discussed in detail in Section 10.6.4. Cu(II) (rf9) and Zn(II) (rf10) coordination compounds are found frequently in enzyme systems where their large reactivity is fully utilized.

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