The relative affinities of ligand atoms for acceptor molecules and ions

Abstract

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

Analyzing the Adsorption Behavior of Thiocyanide on Pure Pt and Ni Electrode Surfaces by Confocal Microprobe Raman Spectroscopy

“…Based on a theoretical calculation, the N end SCN -is assumed to have a lower frequency compared with that of its free ion. 47,48 It is evident that the electrode potential has a significant influence on the frequency of the adsorbed SCN -, which can be 226 ANALYTICAL SCIENCES FEBRUARY 2000, VOL. 16…”

The structure of human carbonic anhydrase II in complex with bromide and azide

“…The fact that these two ligands can replace the zinc water without donating a proton to T199 Oyl might be due to their softness (polarizability), compared to oxygen. Soft anions have a higher affinity for relatively soft cations like zinc and, conversely, hard anions have a higher affinity for hard cations like alkali and alkaline earth metal ions [16]. The higher polarizability of the bromide and azide ions might alleviate the confrontation between its electron orbitals and the lone pair of T199 0~1.…”

The structures of the mixed complexes cadmium(II)–1,4,10,13-tetraoxa-7,16-diazacyclooctadecane diiodide and mercury(II)–1,4,10,13-tetraoxa-7,16-diazacyclooctadecane diiodide

“…Uncomplexed 2,2 is included for comparison . The formation constants in aqueous solution for the first complex between the weak (b)-acceptor (Ahrland, Chatt & Davies, 1958) Cd 2+ and NH 3 and I-respectively are of about the same order of magnitude. With the marked (b)-acceptor Hg 2+ the difference is about four log units in favour of I-(Stability Constants of Metal-Ion Complexes, 1964), indicating a very strong Hg2+-I -interaction.…”