IN 1941 Sidgwick summarised the few available data on the relative affinities of the commoner ligand atoms for various acceptor molecules and ions. Since then the experimental material has increased enormously, and so it seems profitable to attempt a revised and extended correlation involving all the ligand atoms except hydrogen. Admittedly, the quantitative data concerning the heavy donor atoms of Groups V and V I are still sparse, but together with semiquantitative and qualitative evidence there are sufficient to provide a fairly coherent picture. On the other hand, in the case of Group VII where the affinities of the simple halide ions for metal ions can usually be measured conveniently in aqueous solution, the number of quantitative data is now considerable. *
The Relative Co-ordinating A f i t i e s of Ligand Atoms from the SameGroup.-Thcre is no uniform pattern of relative co-ordinating affinities of ail ligand atoms for all acceptor molecules and ions, not even when only simple unidentate ligands of closely analogous structures are considered, e.g., the alkyl derivatives PR,, R,S, etc. Rather, their relative affinities depend on the acceptor concerned. Thus towards trimethyl gallium the relative tendencies of the alkyls of co-ordinating atoms from Groups V and V I to form complexes under comparable conditions are N > P > As > Sb and 0 > S < Se > Te, but towards platinum(I1) the order appears to be N < P > As > Sb and 0 4 S > Se < Te,5 and towards silver N <=< P
> AsOther similarly diverse examples could be given.I n spite of this lack of uniformity kwo regular features have emerged : (1) There is in general a very great difference between the co-ordinating affinities of the first and the second element from each of the three Groups of ligand atoms in the Periodic Table , i.e., between N and P, 0 and S, F and Cl. (2) There are two classes of acceptor : ( a ) those which form their and
Ligands coordinated to acceptors termed ( a ) , or hard, are generally held by bonds of an essentially electrostatic character, while the less numerous group of acceptors termed (b), or soft, form bonds which are markedly covalent.The facts supporting this statement may be summarized as follows [l]: With a certain group of ligands of the same charge, e.g. the halide ions, the complexes formed by (a)-acceptors are invariably stronger, the smaller the ligand. The complexes also become stronger the higher the charge and the smaller the radius of the acceptor involved. For (b)-acceptors on the other hand, the strongest complex is not formed by the smallest ligand of a series, but by a succeeding one. Strong complexes are often formed with uncharged ligands of low polarity, or even no polarity at all, such as olefins. For several elements, the (b)-character increases with decreasing charge of the acceptor, and even becomes most pronounced in the zero oxidation state.For typical (b)-acceptors, the bonds become strongly covalent, especially when the ligands are also very soft, i. e. very prone to covalent bonding. Such ligands as a rule exert only weak electrostatic attraction, and are therefore not coordinated by hard acceptors. Hard ligands on the other hand, i . e . those held mainly by electrostatic forces, are coordinated to all acceptors surrounded by a sufficiently strong field, irrespective of whether they are hard or soft.
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