Orbitals magnetism, lanthanide ions

Three-positive lanthanide ions are not so promising if the objective is building bulk molecular magnets because of the internal character of 4f orbitals, which... 

The lanthanides have electrons in partly filled 4/orbitals. Many lanthanides show colors due to electron transitions involving the 4/orbitals. However, there is a considerable difference between the lanthanides and the 3d transition-metal ions. The 4/ electrons in the lanthanides are well shielded beneath an outer electron configuration, (5.v2 5p6 6s2) and are little influenced by the crystal surroundings. Hence the important optical and magnetic properties attributed to the 4/ electrons on any particular lanthanide ion are rather unvarying and do not depend significantly upon the host structure. Moreover, the energy levels are sharper than those of transition-metal ions and the spectra resemble those of free ions. 

Equation (S6.1) is applicable to the salts of lanthanide ions. These have a partly filled 4f shell, and the 4f orbitals are well shielded from any interaction with the surrounding atoms by filled 5.9, 5p, and 6.9 orbitals, so that, with the notable exceptions, Eu3+ and Sm3+, they behave like isolated ions. For the transition metals, especially those of the 3d series, interaction with the surroundings is considerable. Because of this, the 3d transition-metal ions often have magnetic dipole moments corresponding only to the electron spin contribution. The orbital moment is said to be quenched. In such materials Eq. (S6.1) can then be replaced by a spin-only formula ... 

Contributions from orbital angular momentum cause deviations from these values. For complexes containing heavier metal ions, the interaction of spin and orbital angular momenta is greater. For the lanthanides, the magnetic moment depends on... 

The lanthanide induced shifts (LIS) arise from the interaction of the magnetic nucleus with the magnetic field of the paramagnetic lanthanide ion. This could arise via two mechanisms either a pseudocontact (dipolar) through-space fashion involving a dipole dipole interaction, or by a contact mechanism with delocahzation of unpaired spin density from the lanthanide f-orbitals via the lanthanide substrate bond on to the organic molecule the former is more important. 

The direction and magnitude of the shifts produced by lanthanide ions depends on their magnitude susceptibihty anisotropy terms. Because the unpaired electrons of the lanthanide ions are in 4f orbitals, and these are shielded by filled 5s and 5p orbitals, the magnetic terms are essentially independent of the ion s environment, and remain constant from complex to complex. In organic solvents such as chloroform, rare earths are found... 

The other lanthanides ions do not obey this simple relationship. The 4/electrons are well shielded from the entemal field by the overlying 5s and 5p electrons. Thus, the magnetic effect of the motion of the electron in its orbital is not quenched out. Thus, the magnetic moment must be calculated taking into account both the magnetic moment from the unpaired electron spins and that from the orbital motion. In lanthanides the spin contribution S and orbital contribution L couple together to give a new quantum number J. 

In several aspects, the magnetic and spectral behavior of the lanthanides is fundamentally different from that of the rf-block transition elements. The basic reason for the differences is that the electrons responsible for the properties of lanthanide ions are 4/ electrons, and the 4/ orbitals are very effectively shielded from the influence of external forces by the overlying 5.s2 and 5p6 shells. Hence the states arising from the various 4f" configurations are only slightly affected by the surroundings of the ions and remain practically invariant for a given ion in all of its compounds. 

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