Eigen-Tamm mechanism

EIGEN-TAMM MECHANISM Rate-limiting step in metabolic pathway, PACEMAKER REACTION RATE OF APPEARANCE CHEMICAL KINETICS RATE OF DISAPPEARANCE CHEMICAL KINETICS... 

Because of the overwhelming predominance of electrostatic interactions, the kinetics of lanthanide complexation are relatively rapid and conform to the Eigen-Tamm mechanism(59) ... 

The pressure jump relaxation technique has first been applied with conductometric readout to study the formation of metal complexes involving Be2+, Al ", Ga ", In ", Ni , and Fe , for which the reaction can be described by the Eigen-Tamm mechanism. These measurements have been summarized before. Recently examples have been found where in the case of bidentate ligands the formation of both.monOdentate and bidentate complexes could be observed separately [l3l. Since the different complexes form at different rates, rate as well as equilibrium constants are obtained. In the case of carboxylate ligands it is difficult or even impossible to obtain these equilibrium constants by static measurements. Besides aqueous and nonaqueous solutions also mixed solvents have been used for the investigation of metal complexation [14,15]. In the case of the Be /S0 system which shows a single relaxation effect in aqueous solution, up to five different relaxation effects are observed in mixed solvents, which refer to different composition of the solvation shell of the cation. 

The work being reported in this paper is part of a continuing study on metal-complex formation using BeSO as a test electro lyte. In pure aqueous solution the reaction has been extensively studied (1) and is shown to follow the Eigen-Tamm mechanism ... 

Eigen-Tamm-Wilkins mechanism for complex formation.The same mechanism is thought to operate in the formation of the oxalate complex from cis-[Co(en)2(NH3)(OH2)]. Again all three forms of the ligand, ox , oxH", and 0XH2, react with the aquo complex. The outer-sphere association constant for the oxH" anion is 1.5, for the ox anion 5.8 dm mol ° The kinetic results here are compared with those established several years ago for the reaction of [Co(NH3)5(OH2)] with oxalate.The formation of the sulfito complex [Co(NH3)5(S03)] is mentioned in Section 5.7.7. below. 

The data were analysed in terms of a three-step complex formation mechanism of Eigen and Tamm [18]. The mechanism consists of successive removal of one water molecule from the solvation sheath of each ion in three reversible steps as the ions approach from infinite separation to ultimate contact. 

The advancement of the theoretical description of ion-pairing was marked by the distinction between internal (or contact or tight) and external (or solvent separated, loose) ion-pairs. Eigen and Tamm [63,64] proposed a stepwise formation of the contact ion-pair while the formation of the solvent separated ion-pair is diffusion-controlled, the elimination of the solvent molecnles to form the contact ion-pair was the slowest stage. Ultrasonic absorption data snpported the so-called Eigen mechanism represented in Figure 2.1. 

The coupling constant is inconsistent with carboxyl coordination but consistent with carbonyl coordination 15). Similar data for -ketobu-tyrate 15) and oxalacetate (19) have been fit by exchange contributions (1/tm) and inner sphere contributions Tm and T2m)- The rates of formation of these metal bridge complexes from an outer sphere complex ( 3,4) are limited predominantly by the rate of dissociation of a water molecule from the coordination sphere of the enzyme-bound manganese (Figure 3, Table V) (15,19), as required by the Sj l-outer sphere mechanism of Eigen and Tamm (20),... 

The interpretation of the ultrasound relaxation data is based on the assumption that the rate-controlling step is the loss of water from the cation solvation shell via the Eigen mechanism of complex formation (Eigen and Tamm 1962)... 

The complexation reactions usually proceed by the Eigen mechanism (Diebler and Eigen 1959, Eigen 1963, Eigen and Tamm 1962). This mechanism involves two steps, the rapid formation of an outer-sphere association complex (i.e., an ion pair) and the subsequent rate-determining step in which the ligand displaces one or more water molecules. 

According to the theory of Eigen and Tamm, the mechanism of ligand exchange is given by the following scheme ... 

Finally, we will only mention the ultrasonic absorption relaxation method which is, in fact, several different experimental techniques that permit the determination of relaxation times in the range 10 to 5 X 10 sec. Several of the dissociation field effect results in Table II have been confirmed by this method, and much new data on ligand substitution of alkali metal complexes have also been acquired by ultrasonic techniques (Eigen, 1963). Eigen and Tamm (1962) have thoroughly reviewed the role these techniques have played in revealing the mechanism of fast consecutive substitution reactions in metal complexes. 

---