The Magnetic Properties of Coordination Compounds

The optical isomers of c/s-[CoCI(NH3)(en)2l first isolated by Werner and King. The two N atoms connected by a curved line are cartoon representations for the ethylenediamine ligand. 

The number of unpaired electrons, In turn, can be predicted on the basis of crystal field theory (see Chapter 16) and whether or not the molecule has a high-spin (HS) or low-spin (LS) electronic configuration. The experimental and calculated values of the effective magnetic moments for the first-row transition metals are listed in Table 15.5. 

TABLE 15.5 Calculated and experimental effective magnetic moments for the octahedral first-row transition metals in units of Bohr magnetons. 

The most common way to ascertain the magnetic properties of materials is to measure their molar magnetic susceptibility, The magnetic susceptibility is related to the effective magnetic moment according to Equation (15.10) and is mea-sured using either a Gouy balance or an Evans balance. 

---

Use crystal field theory to interpret the magnetic properties of coordination compounds in terms of the electron configurations of their central ions (Section 8.4, Problems 21-25). 

As discussed previously, the magnitude of the crystal field splitting energy largely determines the number of unpaired electrons in a given compound. This in turn, as we see in this section, has a direct bearing on the magnetic properties of coordination compounds. 

We see, then, that the magnetic properties of coordination compounds are consistent with CFT. Furthermore, these properties can be used to substantiate the spectrochemical series of ligands. 

Confirmation of CFT comes from a consideration of the magnetic properties of coordination compounds. Molar susceptibilities, derived from measurements on a Guoy balance, can be related to the magnetic moment of the complex. A comparison of this experimentally derived magnetic moment with spin-only moments yields a measure of the number of unpaired electrons in the compound. The results derived from a consideration of magnetic properties are consistent with CFT. 

The dinuclear asymmetric [Gd2(Tp)2(DTBSQ)4]CHCl3 consists of two Gd ions, one eight- and the other nine-coordinate. The magnetic properties of this compound have been investigated by magnetic susceptibility measurements and high-field electron param-agnetic resonance spectroscopy. 

We explore how crystal-field theory allows us to explain some of the interesting spectral and magnetic properties of coordination compounds. 

However, the valence bond theory approach to transition metals has severe limitations. It fails to account for the absorption spectra and magnetic properties of coordination compounds. These and other properties are more satisfactorily explained by crystal field theory or ligand field theory. 

Two striking features of many coordination compounds are that they are colored, paramagnetic, or both. How do these properties arise Can we control the color and the magnetic properties of compounds by chemical means To find out whether that is possible, we need to understand the electronic structures of complexes, the details of the bonding, and distribution of their electrons. 

Due to some special structural and magnetic peculiarities, in particular, free-radical properties, the quinone ligands and their metal complexes are apart from the other kinds of coordination compounds [125a, 138a], as will be shown below. Here we present an overview of the main methods for the synthesis of complexes containing benzoquinone, semiquinone, and catecholate ligands, and peculiarities of the products. 

---