Spin block metal ions

In practice, spin-orbit coupling is small for first-row d-block metal ions, so that equation 7.1 may be applied. Furthermore, the d orbitals of a transition metal ion usually show significant interactions with the ligand orbitals. In such cases, the orbital angular momentum contribution is usually quenched so that gS(S + 1) L(L +1). Where this happens, equation 7.1 can be simplified to the spin-only formula shown in equation 7.2 ... 

Paramagnetism arises from unpaired electrons. Each electron has a magnetic moment with one component associated with the spin angular momentum of the electron and (except when the quantum number / = 0) a second component associated with the orbital angular momentum. For many complexes of first row J-block metal ions we can... 

Table 20.8 Calculated magnetic moments for first row

Table 20.9 Spin -orbit coupling coefficients. A, for selected first row d-block metal ions.

An important point is that spin-orbit coupling is generally large for second and third row rf-block metal ions and this leads to large discrepancies between p(spin-only) and observed values of p ff. The complexes cw-[NbBr4(NCMe)2] and cis-... 

For d-block metal ions, eq. 20.20 gives results that correlate poorly with experimental data (Tables 20.11 and 20.12). For many (but not aU) first row metal ions, A is very small and the spin and orbital angular momenta of the electrons operate independently. For this case, the van Vleck formula (eq. 20.21) has been derived. Strictly, eq. 20.21 applies to free ions but, in a complex ion, the crystal field partly or fully quenches the orbital angular momentum. However, in practice, there is generally a poor fit between values of /Ueff calculated from eq. 20.21 and those observed (compare data in Tables 20.12 and 20.11). 

Figure 26.1 illustrates that the rates of water exchange for the t/-block and ions span a much greater range than do those of the group 1, 2 and 13 metal ions. The kinetic inertness of (e.g. [Cr(OH2)6] in Fig. 26.1) and low-spin cf (e.g. [Rh(OH2)6] and [Ir(OH2)6] " ) can be understood in terms of crystal field theory. More generally, the 20 orders of magnimde covered by values of k for the fi -block metal ions follow from the different ncf-electron... 

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