Magnetic properties of transition metal complexes

The magnetic properties of transition metal complexes. B. N. Figgis and J. Lewis, Prog. Inorg. 

Exhaustive compilations of magnetic properties of transition metal complexes are available.166-192-194 The following sections describe very briefly the magnetic behaviour of a selection of complexes or groups of complexes, from the various electronic configurations which arise for the transition metals. They are intended to be illustrative of the points made in the earlier sections no attempt at completeness of coverage is made. More detailed and intensive accounts of magnetic properties of transition metal complexes are available.99 166 169-174-175 186-195-197 No mention is made of results for lanthanoid and actinoid elements, nor of ESR g-values. 

Valence bond theory is somewhat out of favour at present a number of the spectroscopic and magnetic properties of transition-metal complexes are not simply explained by the model. Similarly, there are a number of compounds (with benzene as an organic archetype) which cannot be adequately portrayed by a single two-centre two-electron bonding representation. Valence bond theory explains these compounds in terms of resonance between various forms. This is the origin of the tautomeric forms so frequently encountered in organic chemistry texts. The structures of some common ligands which are represented by a number of resonance forms are shown in Fig. 1-11. 

In the previous sections we have discussed the ligand-field theory from the point of view of quantum chemistry, and have presented an ab initio derivation of the ligand-field Hamiltonian (1-5). In principle this Hamiltonian can be constructed explicitly using the standard techniques of computational quantum chemistry, although in practice it is evident that this would be subject to the usual difficulties encountered with large molecules. Our concern in this section is with the use of Eq. (1-5) as the basis for a parameterisation scheme that permits the interpretation of the spectroscopic and magnetic properties of transition metal complexes in terms that are chemically intelligible. 

Experimental measurements of the optical absorption spectra and magnetic properties of transition-metal complexes provide a critical test of the validity of crystal field theory. The theory makes specific predictions about the strengths of crystal fields produced by different ligands. Charged ligands such as the halides... 

Figgis, B. N. and Lewis, J., The Magnetic Properties of Transition Metal Complexes. . 6 37... 

The magnetic properties of transition-metal complexes can readily be understood in terms of CFT. Transition metal ions have partially filled d orbitals. If these orbitals are degenerate, Hund s rule predicts that unpaired electrons will be present. For example, a metal ion containing three d electrons (called a cf system) should have three unpaired electrons C O ) a metal ion... 

The sections following below serve a dual purpose (a) to introduce, define, and briefly discuss all those quantities and concepts which directly occur in the tables and (b) to provide a short introduction to the review literature on the magnetism of transition metal compounds. For further information on the theoretical and/or experimental aspects of the method, the literature listed in section 1.1.8.3 and 1.1.8.4 of the references may prove helpful. It should be noted that apart from the general references on magnetism (section 1.1.8.1), these sections list only literature which appeared in the years covered by this volume. Therefore, the reader may sometimes want to consult the additional references in volume II/2 of this series. In addition, the review of Figgis and Lewis [5] on the magnetic properties of transition metal complexes is still useful. 

Valence bond (VB) theory can explain the shape and magnetic properties of transition metal complexes, but only at a simple level. It is an unrealistic model since it invokes the use of 4d orbitals in some complexes, such as [Fe(H20)6]. The 4d atomic orbitals are at a significantly higher energy than the 3d atomic orbitals. 

Spin-orbit coupling has to be included in a discussion of the magnetic properties of transition metal complexes whenever a spin-derived magnetic moment and an orbitally-derived magnetic moment coexist. Spin-orbit... 

Because they can, and usually do, remove orbital degeneracies—and thus reduce the orbital contribution to the magnetism—low-symmetry ligand fields cannot be ignored in a study of the magnetic properties of transition metal complexes. Unfortunately, it is no easy matter to determine the splitting effects of low symmetry fields usually it is necessary to work with two... 

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