LETTERS To the Editor: Dr. J. J. Zuckerman [THISJOURNAL, 42, 316 (1965)l points out the dangers of representing the splitting of the d orbitals in an octahedral field by diagram (a) in the figure below and suggests that (b) is a more useful diagram. However (b) may be, and has been,
represented as (c), which-if presented without explauation-is just as misleading as (a) and has the added disadvantage that it obscures, rather than assists, the derivation of crystal field stabiliation energies. In an attempt to avoid this diiculty, may I suggest that Dr. Zuckerman's point about the center of gravity of the d orbitals be further emphasized by separating the energetic stages completely as in diagram (d).
Crystal field splitting
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Spherically symmetric coulombic repulsion
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sponds to a real process in which six or four ligands gather about the central atom at a distance r to produce somehow a spherically symmetric field, then rearrange themselves at some point into one or another of the standard geometries, in doing so removing the degeneracy from the central atom's d orbitals. Norbury's diagram (c) corresponds to a concerted process: simultaneous approach and splitting. I had hoped to combine the advantage of Norbury's (a) and ( c ) in my proposed diagram [Norbury's ( t ) ] . Gas-phase formation of transition metal complexes from the free ion is rare and perhaps even unknown. It is likely that complex formation in solution nearly always utilizes ions already complexed. Thus the changes for these rea: processes might best be represented in yet another type of diagram where changes in the magnitudes of the A terms and changes in the form of the diagrams (to conform to the fields of different ligands and to different geometries respectively) are shown. In either case, to make a truly adequate presentation we should need more detailed knowledge of the mechanisms of formation of ligand-metal bonds than is at present available to us, especially in order to draw correctly the slope and form of the tie-lines which connect the initial and final states of the system. At least the straight lines in the Norbury diagrams should be redrawn as curves in conformity to the laws of simple "billiard-hall" electrostatics for the approach of particles of opposite charge. I have shorn our ignorance of these details by putting wavy tie-lines in my original diagram. The confusion which plagues the use of these diagrams arises from a misunderstanding over whether the d i a grams represent a bookkeeping device for the summingup of energy diierences or a scientific model of the actual reaction processes, or whether they purport to do both. Born-Haber cycles and crystal field splitting diagrams are of the first-mentioned type for almost all except gedankm experiments. This a point worthy of more emphasis in the lecture hall. It would be unfortunate indeed for students to come away with the notion that these diagrams are intended to imply actual reaction paths.
It would be still better to fit these energetic terms into a Born-Haber type cycle wherever possible, so that their relative magnitudes are clear.
To the Editor: Thank you for offering the hospitality of your columns for my comment on the letter of Dr. A. H. Norbury. Norbury's suggestion raises an important point concerning the use of crystal field splitting diagrams. If the abscissa of the splitting diagram is taken to represent the distance between ligand and central atom, r, with decreasing r (approach of ligand) toward the right, then the Norbury diagram ( d ) corre688
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Journol o f Chemicol Education
To the Editor: After our article on "The Graphical Estimation of Thermodynamic Properties" was completed [November issue of THIS JOURNAL, 42, 601 (1965)], an important book was published on the subject of estimation of properties. "Methods of Comparative Calculation of Physical-Chemical Properties," by M. Kh. KarapetJyants, published by the Academy of Sciences of the USSR, Publishing House "Science," Moscow, 1965, is avaluable general reference on this subject and should be included in our bibliography.
