Electronic Structure and Electron Transport Properties of Metal Ions 13 



Gibson, J. F., Ingram, D. J. E. & Schonland, D. (1958). Disc. Faraday Soc, No. 26, 72. 



Grifhth, J. S. & Orgel, L. E. (1957). Quart. Rev. XI, 381. 



Griffith, J. S. (1956). Proc. Boy. Soc. A. 235, 23. 



Griffith, J. S. (1956). J. Inorg. Nucl. Chem. 2, 229. 



Griffith, J. S. & Orgel, L. E., Unpublished calculations. 



Griffith, J. S. (1957). Nature, Lond. 180, 31. 



Griffith, J. S. (1958). Disc. Faraday Soc. 216,91. 



MoFFiTT, W. & Ballhausen, C. J. (1956). Ann. lier. phys. Chem. 7, 107. 



Orgel, L. E. (1956). Proc. lOtli Soliay Conference in Chemistry, Brussels. 



Paultng, L. (1940). The Nature of tiie Chemical Bond, Cornell University Press. 



Taube, H. (1959). Advances in Inorganic and Radio Chemistry 1, 1, Academic Press, 



London, New York. 

 Eraser, R. T. M., Sebera, D. K. & Taube, H. (1959). /. Amcr. chem. Soc. 81, 2906. 



DISCUSSION 



Terminology in Ligand-Field Theory 



Williams: Dr Orgel's paper contains a discussion of the quantity. A, which is used by 

 him both in thermodynamic and spectroscopic calculations. In one place it is sug- 

 gested that there is a critical value of A such that if it is exceeded a change of 

 paramagnetic mom.ent would be observed. A is here used in a thermodynamic argu- 

 ment and is an indication of the field strength in the ground state of a complex. At 

 another place it is stated that extensive studies of spectra have led to the relative 

 values of A ; A is related to an excitation energy difference between two states, the 

 ground and the excited states. What is A ? Is it a parameter of the field or is it only 

 to be correlated with differences in character between excited and ground states, or 

 does it represent a confusion of these two factors ? We shall now indicate why we 

 consider the last statement to be true. 



The order of A given by Orgel for a series of ligands is part of the spectro-chemical 

 series 



I- < Br- < CI- < F- < OH- < H2O < carboxylate- < SCN" < pyridine < NH3 



< NOr < CN-. 



This series is often confused with a series of increasing field strength (Williams, J. 

 chem. Soc. 8 (1956)). The effect of ligands in reducing the paramagnetic moment of 

 ferric complexes (Scheler, Schoffa & Jung, Biochem. Z. (1957) 329, 232) follows the 

 different order 



F- < CI- < H2O < carboxylate- < OCN" < SCN- < OH- < NOr < NH3 



< pyridine < N3- < CN-; 



note particularly the underlined ligands. Again there is no doubt that the stability 

 of metal complexes with different ligands does not follow a given order of ligands. 

 With some cations an order somewhat like the spectrochemical series is observed, 

 with others the order is almost completely reversed. Thus experiment shows that A, 

 obtained from spectra, is not to be used as a guide in discussions of thermodynamic 

 quantities (see Jorgenson, Orgel, Williams, Disc. Faraday Soc, 26, 1958, p. 123-130, 

 110-115, 180-187). 



However, theory (same references as above) also shows why this is so dangerous. 

 The correlation of A with field strength is only apparent in a first order perturbation 

 treatment of a simple electrostatic crystal field model. A second order perturbation 

 treatment introduces polarization of the ligand dependent upon the cation or, if one 

 wishes to put it this way, covalency, and changes in the radial as well as the angular 

 dependences of the d wave-functions aftect the energies of states. This was pointed 

 out by Owen {Disc. Faraday Soc, 19 (1955)). 



