14 Discussion 



This covalency is in part independent of field symmetry and is often called central 

 field covalency. It is best explained by saying that not only is there a tendency for 

 the d electrons of the cation to go into particular directions in space when a field is 

 applied but that they spread out radially in space over the ligands also. The stabihty 

 of a complex depends not only on the ability of a ligand to polarize the d electrons 

 into given directions, where they get out of the way of the ligand electrons, but also 

 on the ability of the ligands to allow the d electrons to spread out over them and the 

 ability of the cation to allow the ligand's electrons to spread over it. Jorgensen has 

 shown that the series of ligands which allow increasingly the d electrons to spread 

 out, is 



F- < H2O < NH3 < L^^^^'g) < SCN- < Pyridine < Cl^ < CN- < Bi- < (I-?) ; 



Note the underlined Hgands. 



This series is different again from the spectrochemical series. Thermodynamic 

 properties such as the stabilities of complex ions and the relative stability of two spin 

 states are just as likely to follow either one of the two series, spectrochemical and 

 nephelauxetic, which are both derived from spectra. The order of ligands which will 

 bring about a change of spin state is even dependent upon the valency state of the 

 cation. 

 Orgel: 1. The procedures used to derive A, the spectroscopic ligand-field strength, and to 

 determine the "nephelauxetic" series are well defined. To suppose that a single A 

 suffices for ground and excited states of a complex is an approximation, but one which 

 is justified by the success of the simple theory in interpreting spectra. If, as Williams 

 suggests, A varies greatly from state to state, the ligand-field theory would never have 

 been adopted, since it would have failed to accommodate even the simplest observa- 

 tions on the spectra of high-field complexes. 



2. The approximations implicit in the treatment of spin-pairing are more serious since 



(a) The internuclear distances decrease on spin-pairing and so the ligand-field 

 increases (this is in contrast to the situation encountered in spectroscopy, where all 

 energies are determined for the same internuclear distance). (See, for example, Orgel, 

 10th Solvay Conference Proceedings, Brussels, 1955.) 



(b) Increased delocalization in the low-spin state facilitates spin-pairing. (See, for 

 example, the many papers on Co+++ in the spin-paired state.) 



It may well be that in special cases these effects can change slightly the ligand-field 

 order; I would only question whether any experimental evidence for this is available 

 in the regular octahedral complexes to which the theory applies. 



3. The series of ligands of Scheler, Schoff"a and Jung may reveal a reversal of the 

 ligand-field parameter. Before concluding that this is so Williams should establish: 



(a) That detailed calculations show that the conclusions of the theory of regular 

 octahedral complexes can be taken over for non-regular complexes. (This is plausible 

 but by no means obvious or easy to demonstrate.) 



(b) That the NOg" group is present as a nitro group (not a nitrito group) in both 

 high- and low-spin forms of ferric haem complexes. 



(c) That the addition of ligands to haems (and changes of pH, etc.) do not affect 

 the protein-metal interactions. 



4. Williams' identification of electrostatic (as contrasted with covalent) theories with 

 first order perturbation theories, and the latter with ligand-field theory, is incorrect. 



In conclusion, I should like to say that I usually find myself less in disagreement 

 with Williams' view than with those which he attributes to others. In presenting 

 material in reviev.'s or introductory articles it is normal to omit the many qualifications 

 which appear in the detailed literature, and to indicate that this has been done. That, 

 for example, is why I say that the theory of spin-pairing as presented is "much- 

 oversimplified" and why I placed pyridine and ammonia, for the purposes of an 

 elementary treatment, together in the ligand series as amines. (The ligand-fields of 

 the compounds are very similar and the order probably does change from one com- 

 pound to another, both for electronic and stereo-chemical reasons.) A useful purpose 



