Octahedral complexes outer sphere mechanism

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

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 ... 

In a later, more detailed study, the outer-sphere complexation of hexahydrated Mil" by FA is confirmed by ascribing the H relaxation mechanism mainly to distortion of the octahedral hydrated-metal symmetry occurring by collisions with water molecules outside the complex (Deczky and Langford, 1978). In the presence of FA, the marked increase observed for the longitudinal relaxation rate dependence on Cu ion concentration compared to Cu -aquo ion and Cu " "-bipyridine complex is attributed to an inner-sphere complex of Cu " " with FA (Deczky and Langford, 1978). 

Most researchers have assumed that electron transfer between Fe3+ and SO " is an outersphere process. However, formation of an inner-sphere complex appears to be a necessary step in this mechanism when open coordination sites are available in the octahedral coordination sphere of Fe(III) (Conklin and Hoffmann, 1988a-c). On the other hand, Fe(2,2 -bipyridine)3 +, which has a fully occupied coordination sphere, oxidizes SO3- via an outer-sphere electron transfer to give SOj with the following rate law ... 

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