Coordination complexes thermodynamic considerations

The second chapter deals with quantum chemical considerations, s, p, d and f orbitals, electronic configurations, Pauli s principle, spin-orbit coupling and levels, energy level diagrams, Hund s mles, Racah parameters, oxidation states, HSAB principle, coordination number, lanthanide contraction, interconfiguration fluctuations. This is followed by a chapter dealing with methods of determination of stability constants, stability constants of complexes, thermodynamic consideration, double-double effect, inclined w plot, applications of stability constant data. 

Simple thermodynamic considerations state that the reduction process is favoured (i.e. more positive cu(ii)/cu(p potential values are obtained) if the electron transfer is exothermic (AH° negative) and if the molecular disorder increases (AS° positive). It is therefore evident that the positive potential value for the reduction of azurin (as well as that of the most blue copper proteins) is favoured by the enthalpic factor. This means that the metal-to-ligand interactions inside the first coordination sphere (which favour the stability of the reduced form over the oxidized form) prevail over the metal complex-to-solvent interactions inside the second... 

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 ... 

The premier issue of the journal Inorganic Chemistry was published in February 1962. Much of the focus of that issue was on classic coordination chemistry, with more than half its research papers on synthesis of coordination complexes and their structures and properties. A few papers were on compounds of nonmetals and on organometallic chemistry, then a relatively new field several were on thermodynamics or spectroscopy. All of these topics have developed considerably in the subsequent half-century, but much of the evolution of inorganic chanistry has been into realms unforeseen in 1962. 

Factors that determine the stability of borohydride complexes have been discussed. 3 Thermodynamic considerations include the ease with which a coordinated [BH4] anion can reduce the metal center, Eq. 10.5 ... 

Makar, G.K.R., Touche, M.L.D., and Williams, D.R. (1976) Thermodynamic considerations in coordination, XXIII, Formation constants for complexes of protons, zinc(II) and acid anions and their use in computer evaluation of a better zinc therapeutical. /. Chem. Soc., Dalton Trans., 1016-1019. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

Consideration of the thermodynamics of a representative reaction coordinate reveals a number of interesting aspects of the equilibrium (Fig. 5). Because the complex is in spin equilibrium, AG° x 0. Only complexes which fulfill this condition can be studied by the Raman laser temperature-jump or ultrasonic relaxation methods, because these methods require perturbation of an equilibrium with appreciable concentrations of both species present. The photoperturbation technique does not suffer from this limitation and can be used to examine complexes with a larger driving force, i.e., AG° 0. In such cases, however, AG° is difficult to measure and will generally be unknown. 

It is significant that the only stable formyl complexes isolated to date (58-60) are coordinatively saturated, which eliminates the possible conversion to a carbonyl hydride without the prior loss of a ligand. The unfavorable thermodynamics of (5) for formyl formation are a necessary consideration in developing schemes for CO reduction by this method. 

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