The Influence of Crystal Fields on Transition-Metal Stereochemistry

The Influence of Crystal Fields on Transition-Metal Stereochemistry 

In this section we shall develop the electrostatic crystal-field theory to a sufficient extent to give an account of its stereochemical implications particularly with reference to the distortions from cubic symmetry which are characteristic of certain groups of transition-metal compounds. The treatment is not meant to be complete and the reader is referred to the many reviews (SI, 72, 97, 102, 106) for detailed references, derivations, qualifications, disputations, and applications. 

According to this theory, a transition-metal ion in a compound or complex is supposed to be subjected to an electrostatic field produced by the molecules and ions in its environment, particularly those in its first coordination sphere. Since the ligands which we consider are invariably either negative ions or polar molecules so oriented that their unshared pairs point directly at the metal ion, the field is always roughly equivalent to that due to a set of negative point charges placed about the metal. The electrostatic theory is concerned with the effect of this field on the orbital energies of the metal d electrons. 

We now consider the distribution of electrons on metal ions in octahedral environments. If only one d electron is present it must clearly occupy one of the more stable set of f2 orbitals. If more than one d electron is present, however, complications may arise for in such cases there are two competing tendencies at work. 

With two, or three d electrons present, both of these tendencies can be satisfied simultaneously by placing the electrons in the different U.g orbitals with their spins parallel. For four to seven d electrons there are two favorable arrangements one which places the maximum number of electrons compatible with the Pauli Principle in the more stable orbitals, and the other which maintains a maximum number of unpaired spins. With more than seven electrons there is again a unique arrangement which gives at the same time the best distribution among the orbitals and the maximum number of unpaired spins. This is illustrated in Table VIII. 

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IV. The Influence of Crystal Fields on Transition-Metal Stereochemistry. 12... 

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