Spin-only magnetism

In some cases, there is a considerable difference between the calculated spin-only magnetic moment and the experimental values. This is especially true for complexes that contain ligands such as cyanide... 

Spin-only magnetic moments for octahedral arrangements... 

Spin only magnetic moment at around 298 K for the compound in the solid State. In solution. Measured in solution. 

Vanadium(II) compounds are usually prepared by reduction of acidic solutions of higher oxidation states. The usual geometry is octahedral and the electronic spectra often show three d-d spin-allowed bands. These spectra, as well as those of tetrahedral and linear complexes, have been reviewed.58 For this cr configuration, a spin-only magnetic moment of 3.87 BM is expected. Slow substitution can be predicted for vanadium(II) complexes but such reactions are not as slow as those for the isoelectronic Crm ions. 

In the extreme class III behaviour,360-362 two types of structures were envisaged clusters and infinite lattices (Table 17). The latter, class IIIB behaviour, has been known for a number of years in the nonstoichiometric sulfides of copper (see ref. 10, p. 1142), and particularly in the double layer structure of K[Cu4S3],382 which exhibits the electrical conductivity and the reflectivity typical of a metal. The former, class IIIA behaviour, was looked for in the polynuclear clusters of copper(I) Cu gX, species, especially where X = sulfur, but no mixed valence copper(I)/(II) clusters with class IIIA behaviour have been identified to date. Mixed valence copper(I)/(II) complexes of class II behaviour (Table 17) have properties intermediate between those of class I and class III. The local copper(I)/(II) stereochemistry is well defined and the same for all Cu atoms present, and the single odd electron is associated with both Cu atoms, i.e. delocalized between them, but will have a normal spin-only magnetic moment. The complexes will be semiconductors and the d-d spectra of the odd electron will involve a near normal copper(II)-type spectrum (see Section 53.4.4.5), but in addition a unique band may be observed associated with an intervalence CuVCu11 charge transfer band (IVTC) (Table 19). While these requirements are fairly clear,360,362 their realization for specific systems is not so clearly established. 

Fig. 23 Effective magnetic moment for octahedral d2 complexes like V(III). Left-. Figgis theory dotted - strong field reference (A = 1), solid - weak field reference (A = 3/2). Complete d2 calculation dash-dot-dot. Effect of the orbital reduction factor dashed - k = 0.4, dash-dot - k = 0 (spin-only magnetism). Inset - enlarged low-temperature window. Right the calculated energy levels (not to scale, values in cm-1)...

The spin-only magnetism can occur so that the Curie law (with some TIP) is almost perfectly obeyed. This is the case of A- or. E-terms well separated from excited ones. Examples are covered by 6Aig of Mn(II), 4A2g of Cr(III), or2Eg of Cu(II). 

If the orbital momentum is completely quenched (spin-only magnetism), the Curie constant can be derived from the total spin quantum number S ... 

It is important to mention that a reduction of ligand field symmetry can strongly alter the magnetic properties of ions If the symmetry of a 3d1 system with an Oh-symmetric ligand field is reduced (e.g., by an orthorhombic distortion), the entire orbital moment will be quenched and spin-only magnetism is observed. 

It is important to observe the applicability limits of the Curie-Weiss law and the implied physical meaning of the Weiss temperature It can only be applied to magnetic materials containing spin-only magnetic centers, and it only applies to magnetically condensed systems (as opposed to magnetically dilute systems) at sufficiently high temperatures fer>/ex). 

The co-ordination chemistry of vanadium(n) has been extended by the preparation and characterization of complexes with aliphatic and heterocyclic amines.365 Intense charge-transfer bands dominate the reflectance spectrum of the dark-red or red-brown V(amine)4X2 complexes (amine = py, / - and y-picoline X = Cl, Br, or I) but assignments of some of the d-d transitions indicate a trans-dihalogeno structure this is consistent with the spin-only magnetic moments ca. 3.87 BM. In contrast, the yellow V(amine)2Br2 complexes obtained with / - and y-picoline exhibit considerable antiferromagnetic interaction indicative of polymeric bromine-bridged structures. 

Manganese with its great variety of oxidation states, each with varying numbers of unpaired 3d electrons, is magnetically interesting but not simple. The theoretical spin only magnetic moments for oxidation states +2, +3 and +4 are in Table 2, and for comparison some experimentally determined moments of some representative compounds are in Table 3. 

Spin-only magnetic moments and their calculation... 

Assuming a pairing energy of 16,000 cm calculate the crystal field stabilization energies (in kj mol ) and spin-only magnetic moments of the following complexes ... 

Calculate the spin-only magnetic moment for the following atoms and ions. (Remember the order of loss of electrons from transition metals described near the end of Section 2-2-4.)... 

For paramagnetic molecules the magnetic moment has two sources, spin and orbital contributions. For transition metal ions (with the exception of Co and Co " ") where there is only a small orbital contribution, the magnetic moment reflects the spin-only term, and expected spin-only magnetic moments in octahedral arrangements for d electrons in biologically relevant transition metals are given in Table 6.2. For the first three... 

TABLE 6.2 Spin-Only Magnetic Moments for Octahedral Arrangements... 

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