Magnetism of complex ions

The main reason that the localized electron model cannot fully account for the properties of complex ions is that in its simplest form it gives no information about how the energies of the d orbitals are affected by complex ion formation. This is critical because, as we will see, the color and magnetism of complex ions result from changes in the energies of the metal ion d orbitals caused by the metal-ligand interactions. 

It is this splitting of the 3d orbital energies (symbolized by A) that explains the color and magnetism of complex ions of the first-row transition metal ions. For example, in an octahedral complex of Co " (a metal ion with six 3d electrons), there are two possible ways to place the electrons in the split 3d orbitals (Fig. 19.23). If the splitting produced by the ligands is very large. 

It is this splitting of the 3d orbital energies (symbolized by A) that explains the color and magnetism of complex ions of the first-row transition metal ions. For... 

Until about 20 years ago, the valence bond model discussed in Chapter 7 was widely used to explain electronic structure and bonding in complex ions. It assumed that lone pairs of electrons were contributed by ligands to form covalent bonds with metal atoms. This model had two major deficiencies. It could not easily explain the magnetic properties of complex ions. 

The L and S values are those from which the / value was formed via the vector coupling rule. These formulae strictly apply only for the magnetism of free-ion levels. They provide a good aproximation for the magnetism of lanthanide complexes, as we shall note in Chapter 10, but provide no useful account of the magnetic properties of d block compounds. 

Another distinct property of mixed complexes which we mention here is the possibility to observe two activation regimes of relaxation of magnetization. One of them corresponds to reversal of magnetization of individual ions, at higher temperatures, and the other is related to climbing over the barrier built from the exchange multiplets of the complex. The coexistence of these two relaxation regimes has been recently revealed in the Co Dy ,11 complex in a combined study... 

The formation of complex ions is an important problem for the study of the structure and properties of molten salts. Several physicochemical measurements give evidence of the presence of complex ions in melts. The most direct methods are the spectroscopic methods which obtain absorption, vibration and nuclear magnetic resonance spectra. Also, the formation of complex ions can be demonstrated, without establishing the quantitative formula of the complexes, by the variation of various physicochemical properties with the composition. These properties are electrical conductivity, viscosity, molecular refraction, diffusion and thermodynamic properties like molar volume, compressibility, heat of mixing, thermodynamic activity, surface tension. 

Although the localized electron model can account in a general way for metal-ligand bonds, it is rarely used today because it cannot predict important properties of complex ions, such as magnetism and color. Thus we will not pursue the model any further. 

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