Publisher Summary

This chapter explains that inorganic chemistry has tended to lag behind organic chemistry in the determination of reaction mechanisms. The foundations and concepts of the mechanisms of reactions in solution were laid down between 1920 and 1945 and applied almost solely to the reactions of tetrahedral and planar carbon centers in organic compounds. Organic chemistry in the 1920s period was ripe for the investigation of reaction mechanism. The situation in the inorganic area was less satisfactory. There was no background of systematized reactions and planned synthetic pathways. Preparative inorganic chemistry was a rather haphazard and intuitive discipline. In addition, the view was widely held that virtually all inorganic reactions were either very rapid or unselective. The products were determined by thermodynamic and solubility considerations, unlike those of organic reactions that were normally determined by kinetic and hence mechanistic factors. The few published studies of inorganic reactions in solution were initially motivated by problems somewhat removed from mechanism such as the study of salt effects, optical rotatory dispersion, stereochemical change, and the application of new or unusual methods to the study of reaction rates.

(a) Organic compounds often undergo reaction at one centre, while all other bonds remain intact.

(b) The products of these reactions are generally kinetically controlled and so an indication of mechanism can be gained by a comparison of reactants and products.

(c) A great deal of information about interesting reactions was already available from preparative organic chemistry.

Inorganic Mechanisms [1–28]

The bulk of the information about inorganic reaction mechanisms has been accumulated over the past thirty years. Initially a somewhat undue emphasis was placed on the reactions of cobalt(III) and platinum(II) complexes, but in recent years the chemistry of other transition elements whose reactions are slow has been extensively studied. The development of readily available techniques for the study of rapid reactions has allowed kinetic studies to be extended to the rapidly reacting systems, and few areas of inorganic chemistry remain which are not amenable to some form of kinetic study.

Inert and Labile Complexes

The terms inert and labile are kinetic terms relating to the rates of ligand exchange in metal complexes. Metal ions exist as aqua complexes in aqueous solution, as a result water exchange rates are very important in areas such as metal complex formation and some redox reactions. Complexes which undergo rapid ligand exchange (t1/2<1min) are said to be kinetically labile. Complexes which react more slowly are kinetically inert. It is important to recognise that the terms inert and labile refer to rates of reaction and should not be confused with the terms stable and unstable which refer to thermodynamics. This point can be illustrated by two examples. The formation constant of [Ni(CN)4]2− is very large (log β4 = 22), and the complex is very thermodynamically stable. However, the rate of exchange of CN−

2+(aq)+4CN−⇌Ni(CN)42−β4=[Ni(CN)42−]/[Ni2+][CN−]4 (1.3)

ions with isotopically labelled CN− added to the solution is extremely rapid, indicating that the complex is kinetically labile undergoing rapid ligand exchange. The point can be illustrated by the free energy profile, Fig. 1.1; there is no relationship between ΔG and ΔG†.

A further example is provided by the [Co(NH3)6]3+ ion which will persist for days in acidic solution due to its kinetic inertness, despite the fact that it is thermodynamically unstable as the following equilibrium shows

Co(NH3)6]3++6H3O+⇌k[Co(H2O)6]3++6NH4+;K=1025

Octahedral Complexes

In the first transition series, the kinetically inert octahedral complexes, are generally those with a d3, and low spin d4, d5 and d6 configurations. Typical examples are Cr(III) (d3) and Co(III) (L.S. d6) (other than [CoF6]3− all octahedral cobalt(III) complexes are low spin).

Complexes of Cr(III) and Co(III) normally undergo ligand replacement reactions with half lives of the order of hours, days or even weeks at 25°C. For example, the reaction

Co(NH3)5Cl]2++H2O→k[Co(NH3)5OH2]3++Cl−

has k = 1.7 × 10−6 s−1 at 25°C (t½ = ln2/k =4.08 × 105 s = 283hr). As a result Co(III) and Cr(III) complexes provide convenient systems for detailed kinetic and mechanistic study. The d3 and low spin d4, d5 and d6 configurations have substantial crystal field stabilisation energies (CFSE’s). Plots of the CFSE for high spin and low spin configurations in units of Δ are shown in Fig. 1.2. The magnitude of Δ depends upon the ligands and also on the oxidation state of the metal.

Thus for M(II) ions of the first transition series Δ usually varies from about 90 to 145 kJ mol−1 (7500-12000 cm−1) while for M(III) ions Δ varies from 170-300 kJ mol−1 (14000-25000 cm−1). Such generalisations are less applicable to second- and third-row transition metals, but normally Δ increases by 30-40% on moving from the first to the second row, and by 30-40% on moving from the second to the third row. The magnitude of Δ is also dependent on the symmetry and geometry of the ligand field. For example, Δ is less for tetrahedral than for octahedral complexes with the same donor atoms (Δtet∼4/9 Δoct), Figures 1.3 and 1.4.

Figures 1.5 and 1.6 illustrate the d electron configurations which are independent of the value of Δ, and those dependent on the strength of the octahedral field.

The inert metal complexes such as Cr(III) (CFSE = −1.2Δ) and Co(III) (CFSE = −2.4Δ) have large crystal field stabilisation energies. In the case of Co(III) with six nitrogen donors the CFSE is ca. 250 kJ mol−1. Energies of this magnitude compare with the values of ΔH† for ligand exchange processes, thus for the reaction,

Co(NH3)5Cl]2++H2O→[Co(NH3)5OH2]3++C−ΔH†=96 kJ mol−1.

It would thus be expected that ligand field effects...