Octahedral complexes thermodynamic stability

The thermodynamic stability of the binuclear site has been demonstrated by the spontaneous assembly of [Fe20(02CR)2L2] (13) from ferric salts in the presence of water, an alkyl carboxylate salt, and a tridentate nitrogen donor ligand L that can cap an octahedral face on iron (8). Suitable ligands include tris(pyrazolyl)borates and 1,4,7-triazacyclononanes. Structure (13) is in essence a portion of the basic ferric acetate structure. The complexes are excellent physical and structural models of the diiron sites and model some aspects of reactivity including redox activity and interconversion of the oxo and hydroxo bridge. 

Let us finally consider implications of these findings for reaction mechanisms in metalloproteins. Therefore, we must take into account that, much like with Sabatier s approach, considerations about thermodynamic stability, which might go as a static phenomenon if it were not for the fact that chemical equilibrium is nothing but the ratio of forward and reverse reaction rates, hence it also is about dynamics and might be compared to other reaction rates, this approach being encouraged by the well-known structure-reaction rate relationships for both (at least benzenoid aromatic) substrates and square-planar or octahedral coordination complexes ... 

Substitution Reactions in Square Planar Complexes Thermodynamic and Kinetic Stability 547 Kinetics of Octahedral Substitution 548 Mechanisms of Redox Reactions 557... 

Despite the considerable structural variation found in the siderophores, their common feature is to form six-coordinate octahedral complexes with ferric ion of great thermodynamic stability. The ligating groups usually contain the oxygen atoms of hydroxamate (a) or catecholate (b) anions. 

In summary, some coordination compounds are kinetically inert, whereas others turn out to be labile. Furthermore, this lability seems to be unrelated to the thermodynamic stability of the compound. Now, being a veteran chemistry student trained to ask critical questions, you are about to ask How can we tell which complexes will be inert and which will be labile As you might suspect, this is indeed a crucial question. It turns out that complexes of thefirst-row transition metal ions, with the exception of and Cr , are generally labile, whereas most second- and third-row transition metal ions are inert. But how, you ask, do we explain such a statement Why, for example, should the rates of reactions involving Co " and Cr be different from those involving other first-row transition metal atoms and cations What is it about these particular cations that makes them so inert To start to answer such queries, we now turn to a discussion of some of the most extensively studied reactions of coordination compounds, those involving the substitution of octahedral complexes. 

It has already been stated that chromium complexes of tridentate metallizable azo compounds occupy their position as the single most important class of metal complex dyestuffs because of their high stability. It should be noted, however, that in this context the term stability is not used in the thermodynamic sense but relates to the kinetic inertness of the complexes.25 Octahedral chromium(III) complexes have a tP electronic configuration and the ligand field stabilization energy associated with this is high.26 Ligand replacement reactions involve either a dissociative... 

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