HSAB complex formation

Equation (3) has good quantitative predictive power and is a successful extra-thermodynamic relationship like the Hammett sigma function. No other approach to modeling complex formation equilibria, including HSAB itself, can predict log values for unidentate ligands so accurately. 

Both steric and electronic effects contribute to the magnitude of the formation constant. The effect of sterics are often assessed with the use of molecular mechanics calculations (Chapter 6) and electronic effects, by studying linear free energy relationships— the relationship between free energies and rates of complex formation. HSAB and CFT (or LFT) are used regularly to predict and rationalize metal complex stability. 

The HSAB principle is qualitatively useful, but lacks a satisfactory quantitative basis. Pearson has pointed out that the hard-hard or soft-soft matching of acid and base represents a stabilization that is additional to other factors that contribute to the strength of the bonds between donor and acceptor. These factors include the sizes of the cation and donor atom, their charges, their electronegativities and the orbital overlap between them. There is another problem. Complex formation usually involves ligand substitution. In... 

Cyanogen (1) radicals were reacted with alkanes and reactivities were examined for the temperature range from 170 to 740 Complex formation with AsFs and SbFs was observed and discussed on the basis of the HSAB principle ... 

Again, the much weaker complexation of Rf by the fluoride ion is apparent compared to that of its homologs. A qualitative explanation comes from the Hard Soft Acid Base (HSAB) concept [61, 62]. The small fluoride anions are hard donors and prefer the smaller acceptor ions Zr + and Hf +. The much larger (softer) Rf acceptor ion tends to prefer larger (softer, more polarizable) donor ligand ions like Cl see [45] and Hydrolysis and Complex Formation for a theoretical discussion of fluoride complexation. 

These correlations mean that the HSAB principle could be a useful approach to evaluate the geochemical behavior of metals and ligands in ore fluids responsible for the formation of the epithermal vein-type deposits. Among the ligands in the ore fluids, HS" and H2S are the most likely to form complexes with the metals concentrated in the gold-silver deposits (e.g., Au, Ag, Cu, Hg, Tl, Cd), whereas Cl prefers to form complexes with the metals concentrated in the base-metal deposits (e.g., Pb, Zn, Mn, Fe, Cu, and Sn) (Crerar et al., 1985). 

It should also be mentioned here that many of the chemical reactions which have been "explained with the HSAB model (2) occur in polar solvents and many involve the formation of ionic species. Thermodynamic cycles can be constructed for these reactions which show how many different kinds of effects are operative. When one considers that much of the data involve rate constant and equilibrium constant measurements, the explanation of this data becomes even more complex for there are entropy terms as well as enthalpy terms for all the steps in any cycle that is constructed. Even less information is available concerning these steps than we had above for the coordination model yet explanations are offered based solely on one step (4) — the strength of the bonding. 

Using the HSAB principle, one can rationalize the corrosion inhibition of iron and aluminum by phosphate in which iron phosphate and aluminum phosphate are produced. Ferric and Al3+ are hard acids, and they react with phosphate, a hard inhibitor and give corrosion protection. Corrosion inhibition of Cu2+ and Zn2+ by amines can be rationalized by the formation of amine complexes of Cu2+ and Zn2+, and this is in accord with the principle that Cu2+ and Zn2+ are borderline acids reacting with amines which belong to borderline inhibitors. Corrosion protection of copper (soft acid) by mercapto-benzothiazole (soft inhibitor) is also in keeping with the HSAB principle. 

One of the most interesting applications of the HSAB concept consists in the prediction of the stability of the complexes formed owing to interaction of alkali metal halides with rare-earth metal halides. These systems are of great interest for the materials science of scintillation materials the said complex halides are now considered among the most promising scintillation detectors and sensors. Besides, the Li- and Gd-based materials are especially convenient as effective detectors of thermal neutrons. The compositions and stability of the formed compounds depend considerably on the kind of acids and bases from which the compound is formed. So, Li+ cation is one of the hardest cation acids, and, therefore, the formation of stable complex halides of Li and lanthanides according to reaction ... 

This reaction is shifted appreciably to the right, compared with the interactions with other halide ions. This fact is explained in the context of the HSAB concept the formation of complexes of A1 with Cl-, Br-, or I- ions is less favourable than of the complex fluoroaluminates, since fluoride ion is the hardest halide base and Al3+ ion is referred to as the strongest hard acids. The F- and O2- ions seem to possess closely similar hard basic properties in molten salts. 

Textbooks refer to Chatt s class (a) and class (b) acceptors, a concept subsequently developed in Pearson s HSAB principle. In contrast to this perceived wisdom, the complex [Col3(SbPh3)2], which contains a hard metal centre and extremely soft iodide and SbPh3 ligands, is quite stable.Its formation is thought-provoking. 

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