Transition metal complexes magnetic properties

Obviously, in a presentation such as this, there is not room to develop the basics of the various forms of ligand field theory in detail nor to describe applications to all the relevant physical properties. This chapter will set out to compare the major aspects of the different forms, will give an account of their use in the interpretation of spectra and magnetism of transition metal complexes, and will make some mention of other areas. 

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

The magnetic properties of polynuclear transition metal complexes. P. W. Ball, Coord. Chem. Rev., 1969, 4, 361-383 (150). 

Stable Mn(HI) compounds, Mn(R2r fc)3, have been known for a long time (42, 46). The structure of Mn(Et2C tc)3 is elucidated (47). The inner geometry of the Mn(CS2)3 core does not conform to the usual D3 point symmetry of transition metal complexes of this type, but shows a strong distortion attributed to the Jahn-Teller effect. The electronic spectrum (48, 49) and the magnetic properties of this type of complexes are well studied (50). 

Griffith, J.S. 1956. On the stabilities of transition metal complexes. II. Magnetic and thermodynamic properties. J. Inorg. Nucl. Chem. 2 229-236. 

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible and, for many transition metal complexes, the near infrared region of the electromagnetic spectrum. Spectroscopic methods that probe these electronic transitions can, in favourable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

For quantum chemistry, first-row transition metal complexes are perhaps the most difficult systems to treat. First, complex open-shell states and spin couplings are much more difficult to deal with than closed-shell main group compounds. Second, the Hartree—Fock method, which underlies all accurate treatments in wavefunction-based theories, is a very poor starting point and is plagued by multiple instabilities that all represent different chemical resonance structures. On the other hand, density functional theory (DFT) often provides reasonably good structures and energies at an affordable computational cost. Properties, in particular magnetic properties, derived from DFT are often of somewhat more limited accuracy but are still useful for the interpretation of experimental data. 

There are some molecules in which different parts of the molecule may have approximate local permutational symmetries. Polynuclear transition metal complexes are an example of one class of compounds which may be so treated. Typically the metal atom orbitals on different atoms may be considered to be only slightly interacting, and approximate local permutational symmetries occur. There will, however, be mixing of these different local permutational symmetries to give states of different total permutational symmetry. Properties, as the magnetic moment of43,173 the polynuclear complex, will then be modified over the case in which the local permutational symmetry is considered to be exact. 

Numerous physical properties are studied with transition metal complexes as the subjects, and many of them involve the use of ligand field theory in their interpretation. They vary from those such as the spectra and magnetic properties, which are heavily dependent on ligand field theory, to others such as reaction kinetics where the application of the theory is rather peripheral. 

Although the physical basis of the crystal field model is seen to be unsound, the fact remains that, in summarizing the importance of the symmetry of the ligand environment, it qualitatively reproduces many of the features of the magnetic and spectral properties of transition metal complexes. This early qualitative success established its nomenclature in the fields of these properties. While we shall have little more to say about crystal field theory as such, much of the rest of this article will be couched in the language of the crystal field model, and for that reason some little trouble has been taken to outline its development. 

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. 

Transition metal complexes are interesting as bio-inorganic model systems [155-157] and also because of their material properties (conductivity, magnetism, porosity) and as potential hosts for a variety of guests [156-161]. Whereas salt-like 2D-Cun-coordination polymers are well documented [158-162], far less is known about their neutral counterparts. 

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