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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

Chatt and Veuanxi. 4735 955. Olefin Co-ordination Compounds. Part VI.* Dime Complexes of Rhodium(~).

N3-Ligdnd constant, PL/V ( & 0.05 V cstimated error) + 1.46 1.40 0.00 -0.07 -0.10 -0.35 -0.38 -0.40 -0.4:) -0.58 -0.59 -0.77 -0.78 -0.88 -1.00 -1.16 -1.15 -1.17-1.19 -1.22 -1.26 -1.55measured and values which we have deduced by intcrpolation from data from other series of Complexes (see below). These ligand constants refer t o L as a twoelectron donor ligand which must be added t o the 16electron moiety to give the closed-shell 18-electron complexes [M,L], x bonding being ignored. Considered in this way the nitride ligand in [M,N] is formally N+, the nitrosyl NO+, and the hydride H-. From Table 1 it can be seen that charged ligands occupy the extremes of the scale. The weakest Q donors with strong x-acceptor