Ligand substitution mechanisms 17-electron complexes

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]. 

A different mechanism is operative with the 16-electron complex RuCl2(PPh3)3 (24) (Scheme 20.11). Here, the dichloride complex (25) is rapidly converted into a dihydride species (26) by substitution of both chloride ligands with alkoxides and subsequent eliminations similar to the conversion of 18 to 20 described above [46, 47]. Subsequently, the ruthenium dihydride species 26... 

A Co(IH) complex is inert in ligand-substitution reactions, and its uniform structure is thus maintained even in an aqueous solution. The reaction mechanism of a Co(III) complex in solution is well known, so that a pendant-type polymer-Co(IU) complex, e.g. 17,19, is one of the most suitable compounds for a quantitative study of the effects of a polymer ligand on the reactivity of a metal complex. The reactivities of the polymer-Co(III) complexes are discussed here kinetically and compared with those of the monomeric Co(III) complexes in the following reactions electron-transfer reactions between the polymer complexes and Fe(II) [Eqs. (5) and (6)], and the ligand-substitution reaction of the polymer-Co(III) complex with hydroxy ions or water [Eqs. (7) and (8)J. One of the electron-transfer reactions proceeds via... 

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 ... 

There are other reactions of transition metal complexes which are relevant to our observations on the ac electrolysis. Recently, new mechanisms of ligand substitution reactions have been reported which are characterized by electron transfer reactions as key steps although the overall reactions are not redox processes, e.g.,... 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

The electron theory of Lewis made a considerable contribution in understanding not only reaction routes, but also reaction mechanisms with participation of Lewis acids and bases [20,31,50]. In particular [31], substitution (exchange) reactions of ligands in octahedral complexes include the acid-base interaction (1.1). Oxidative addition reactions can occur when a complex behaves simultaneously as a Lewis acid and a Lewis base [the metal provides electrons for ligand binding and has vacant coordination sites to accommodate two additional ligands, Scheme (1.10)] [34b] ... 

The competing /3-hydrogen elimination and oxidative substitution of the acetoxyalkylpalladium(II) intermediate bear many similarities to the competing oxidative elimination and oxidative substitution mechanisms observed in electron transfer reactions of alkyl radicals with Cu(II) complexes.633,64 An alternative explanation for the competing pathways in the decomposition of the acetoxyalkylpalladium(II) intermediate can be represented by oxidative elimination versus 1-electron transfer followed by a subsequent electron or ligand transfer, that is,... 

Quantitative studies of ligand substitution at 19-electron centers are hard to find. A study of CO substitution in the 19-electron complex 10 established138 a dissociative mechanism and the reactivity order 10 10+. 

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