Magnetic properties of complex ions

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 theories of chemical bonding that were so useful in earlier chapters do not help much in explaining the characteristic colors and magnetic properties of complex ions. In transition metal ions, we need to focus on how the electrons in the d orbitals of a metal ion are affected when they are in a complex. A theory that provides that focus and an explanation of these properties is crystal field theory. 

In 2005, Matsuda and co-workers [20] investigated the magnetic properties of complex 93. The magnetic susceptibility measurement reveals the Mnm ion in the low-spin state (d45 = 1). However, the jJ.es at room temperature is 3.28pB, which is considerably higher than the calculated spin-only value of 2.83 pB for 5 = 1. The extraordinary pe i value should be due to the spin-orbit coupling effect. 

Such effective changes are also manifest in connection with magnetic properties. On the basis of the orbital angular momentum of d-electrons, as will be examined in detail in due course, the magnetic behaviour of complexes is predicted by CFT to depart in a number of ways from that expected for the presence of electron spin alone. In fact, the magnetic properties of complexes are often rather close to the spin-only behaviour,2 28 29 and it is seen that a free-ion description of the J-orbitals is not adequate. 

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. 

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. 

From the valence bond point of view, formation of a complex involves reaction between Lewis bases (ligands) and a Lewis acid (metal or metal ion) with the formation of coordinate covalent (or dative) bonds between them. The model utilizes hybridization of metal s, p, and d valence orbitals to account for the observed structures and magnetic properties of complexes. For example, complexes of Pd(ll) and Pt(Il) are usually four-coordinate, square planar, and diamagnetic, and this arrangement is often found for Ni(II) complexes as well. Inasmuch as the free ion in the ground state in each case is paramagnetic (d, F), the bonding picture has to... 

The spin-orbit coupling constant plays a considerable role in determining the detailed magnetic properties of many ions in their complexes, for example, the deviations of some actual magnetic moments from spin-only values and inherent temperature-dependence of some moments. All studies to date show that in ordinary complexes the values of A are 70-85% of those for the free ions. It is possible to get excellent agreement between crystal field theory predictions and experimental observations simply by using these smaller A values. 

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