Metal ions solvent molecule exchange rate

The definition of solvent exchange rates has sometimes led to misunderstandings in the literature. In this review kjs 1 (or fc2lsolvent]), sometimes also referred to as keJ s 1, is the rate constant for the exchange of a particular coordinated solvent molecule in the first coordination sphere (for example, solvent molecule number 2, if the solvent molecules are numbered from 1 to n, where n is the coordination number for the solvated metal ion, [MS ]m+). Thus, the equation for solvent exchange may be written ... 

Where solvent exchange controls the formation kinetics, substitution of a ligand for a solvent molecule in a solvated metal ion has commonly been considered to reflect the two-step process illustrated by [7.1]. A mechanism of this type has been termed a dissociative interchange or 7d process. Initially, complexation involves rapid formation of an outer-sphere complex (of ion-ion or ion-dipole nature) which is characterized by the equilibrium constant Kos. In some cases, the value of Kos may be determined experimentally alternatively, it may be estimated from first principles (Margerum, Cayley, Weatherburn Pagenkopf, 1978). The second step is then the conversion of the outer-sphere complex to an inner-sphere one, the formation of which is controlled by the natural rate of solvent exchange on the metal. Solvent exchange may be defined in terms of its characteristic first-order rate constant, kex, whose value varies widely from one metal to the next. 

The only divalent main group ions where exchange rates and activation parameters are experimentally available are beryllium(II), which is also the smallest metal ion (rM = 27pm) (28,29) and magnesium(II) (r-yj = 72 pm). An important consequence of this substantial difference in ionic radii is that Be2+ forms predominantly tetrahedral complexes (30) and Mg2+ is in general surrounded by six solvent molecules forming octahedral complexes (31). 

Replacing several solvent molecules on di- and trivalent transition metal ions by non-leaving ligands can have dramatic effects on the solvent exchange rates of the remaining solvent molecule(s) (Tables X, XI, XII and XIII (70,71,83,86-88,92,95,96,146-166) (115,119-121,167-181) (125, 128,129,167,168,181-193)). For example replacing three MeCN solvent... 

Mn (0.97, 0.81) Co (0.72) Mg (0.86), and Al (0.58). The solvent exchange rates (Kex S 1) for inner-sphere water molecules in metal ions are Al(10°) Mg (105) Co (105 5), and Mn(106 7) [21].The order of increasing rate constants in acidic solution is, Al Ligand exchange rates take on special importance for Al because they are slow and the system may not be at equilibrium. Mg, Mn, and Co have around 105 faster exchange rate over Al. These differential characteristics of the metals play a crucial role in metal-DNA interactions. 

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