A and B Trends in Complex Chemistry

The natural history of chemistry consists since many centuries in empirical rules about the relative affinity of various elements. Though metathetical reactions are frequently used in preparation, the main origin is the filtering of insoluble precipitates, as when an aqueous solution of silver sulphate and another of barium iodide leave almost pure water in the filtrate  

However, comparable exchange of neighbour atoms (ligating atoms) can occur in the gaseous state 

some hetero-atomic bonds must be considerably stronger than the homo-atomic bonds in the elements. Linus Pauling considered this tendency to be so fundamental that the defined the thermochemical electronegativity x from the relation of bond strengths (AH0 values)/ for dissociation to gaseous atoms 

Schwarzenbach developed complexometric titrations with multidentate ligands such as nitrilotriacetate N(CH2C02)i3 and the potentially sexidentate (two N and four 0) ethylenediaminetetra-acetate and potentially octadentate (three N and five 0) diethylenetriaminepenta-acetate. Such synthetic amino-acids form strong com- 

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