Ligand substitution mechanisms complexes

Although direct complex formation is observed kinetically (stopped flow) and spectrophotometrically, where X = Br or Cl, the reaction with I results in an oxidation of the halide. The reactions are rapid and there is the question of inner- or outer-sphere electron transfer, for the [14]aneN4 complex. However, further studies (140) using ligand substituted (dimethyl) complexes reveal that for the rac-Me2[14]aneN4 isomer, two processes are observed, k = 2.9 x 104 M-1 sec-1 and a subsequent redox step, krci = 5.5 x 103 M-1 sec-1, both of which are iodide dependent. The mechanism proposed involves the formation of an octahedral complex which further reacts with a second mole of I- in the redox step ... 

Although quantitative studies of substitution reactions have only been launched quite recently, a considerable body of experimental data is available, so that theoretical principles of ligand substitution mechanisms in classical inorganic complexes have been developed. Unfortunately the same cannot be said for substitution reactions involving 7r-bonded hydrocarbon ligands, in spite of a continuously expanding number of publications in this field. In the majority of studies substitution reactions are used to obtain novel transition metal 7r-complexes, and a far lesser number of papers deal with the quantitative aspects of the exchange. 

Information on ligand substitution mechanisms should aid us to understand more profoundly homogeneous catalysis by transition metal complexes, where probably consecutive substitution and transfer reactions of ligands from metal to a substrate and back take place continuously. 

For a vibronically relaxed bound ES, ligand substitution mechanisms can be discussed in terms of models developed for analogous thermal reactions [36. The limiting mechanisms would be the dissociative (D) and associative (A) pathways, where the rate-limiting steps are, respectively, dissociation of the M-X bond or formation of the M-Y bond to form distinct intermediates (Eqs 6.16 and 6.17). The electronic nature of such intermediates is ambiguous, since these species may also be electronic excited states. For example, the cis to trans isomerization concomitant with the photoaquation of Cl from the Rh(lII) complex cis-Rh(NH3)4Cl2 was successfully explained by a model where Cl dissociation gave a pentacoordinate intermediate in a triplet LF excited state [37, 38]. 

Figure 4.16 Structure and ligand substitution mechanism for the complexes [Rh(diolefin) (PN3)]BF4.

The data are consistent with an I4, mechanism for ligand substitution. The complex [Fe(cat)]+ undergoes a subsequent slow electron-transfer reaction. Under conditions of higher hydrogen-ion concentration, however, 1.0>[H+]>0.1 mol 1, the redox process may be studied using the stopped-flow method. In this acidity range, complex formation (I) is effectively suppressed and the mechanism proposed involves the intermediacy of semiquinone radicals,... 

The electrochemical behavior of niobium in different types of molten electrolytes and the influence of ligand substitution in niobium-containing complex ions on the reduction mechanism is comprehensively reviewed by Polyakov [555]. 

In conforming to an expected linear free energy relationship, the Ce(lV) oxidation of various 1,10-phenanthroline and bipyridyl complexes of Ru(II) in 0.5 M sulphuric acid are consistent with the requirements of the Marcus treatment . The results for the oxidation of the 3- and 5-sulphonic-substituted ferroin complexes by Ce(IV) suggest that the ligand does not function as an electron mediator, and that the mechanism is outer-sphere in type. Second-order rate coefficients for the oxidation of Ru(phen)j, Ru(bipy)3, and Ru(terpy)3 are 5.8x10, 8,8 X 10, and 7.0 x 10 l.mole . sec, respectively, in 0.5 M H2SO4 at 25 °C a rapid-mixing device was employed. 

Skibsted, L.H. (1986) Ligand substitution and redox reactions of gold(III) complexes. Advances in Inorganic and Bioinorganic Mechanisms, 4, 137-183. 

Thus, although the rate of substitution should be very dependent on the nature of Lx, distinguishing the operation of an A mechanism according to Eq. (8) from ligand substitution proceeding through an outer-sphere complex on the basis of rate laws is usually not feasible. This does not, however, preclude the operation of an A mechanism within an outer-sphere complex. 

---