Class metal complexes, formation with stability

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

By comparing the stability constants relative to different polymers, and models, it may be observed that they increase with the number of basic nitrogens present in the repeating unit. Furthermore, the stability constants of the Cu2+ complexes of the polymers of the first class are higher than those of the isomeric polymers of the second class. This fact, and the lower d-d energy band of the complexes of the second class, has been explained with the lack of partecipation in the latter of the C = O groups to the metal coordination l02). Viscosity measurements show that for the complexes with polymers of the first class, the viscosity monotonously decreases upon increasing the pH until the formation of the complex CuL is complete, and then remain nearly constant. On the contrary, in the case of the polymers of the second class, risp/c is... 

Ruthenium s supremacy in the carbene chemistry of Group 8 elements is a direct consequence of the tremendous interest raised by NHC-Ru complexes as catalysts for olefin metathesis. Indeed, the synergy of a late transition metal tolerant of a wide variety of functional groups, together with a class of ligands whose physical and chemical properties are easily modulated to tailor the activity, selectivity, stability, water-solubility, recoverability, or latency of the resulting catalytic species, translated into an unprecedented success story of modern synthetic chemistry. Yet, the ability of ruthenium complexes to promote carbon-carbon bond formation goes well beyond... 

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