High-spin transition metal ions

For axially symmetric complexes, the parameter E is zero, and the spin functions S,ms) are eigenfunctions of the spin Hamiltonian  

For example, consider a d3 Cr(m) complex in an axial ligand field with g= 1.98, E = 0.0455 cm-1, E- 0. For the magnetic field along the molecular z-axis, the energies are  

Transitions among these levels have intensities proportional to the square of the matrix element of Sx. These are easily found to be  

When the field is along the z-axis, transition intensities are proportional to the square of the Sz2 matrix element. The. S -2 matrix for B = 1000 G is  

The 1 - 4 transition is only weakly allowed compared with the 1 - 2, 2 - 3, and 3 - 4 transitions however, it is often observed, particularly in powder spectra since it tends to be considerably sharper than the other transitions. Notice that the 1 - 3 and 2 - 4 transitions are still forbidden. Since the wave functions are field-dependent, the Sz matrix elements also depend on the field. Thus the observed 1 - 2, 2 - 3, and 3 - 4 transitions would be different than predicted from the Sz2 matrix at 1000 G. 

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Figure 3.5. The stability constant for high-spin divalent metal ions of the first transition series coordinated by six monodentate ligands. The dashed line gives the expected increase in K without the contribution from the nonbonded valence electrons on the metal ion. The characteristic double hump is the result of the crystal-field effect.

Shannon and Prewitt base their effective ionic radii on the assumption that the ionic radius of (CN 6) is 140 pm and that of (CN 6) is 133 pm. Also taken into consideration is the coordination number (CN) and electronic spin state (HS and LS, high spin and low spin) of first-row transition metal ions. These radii are empirical and include effects of covalence in specific metal-oxygen or metal-fiuorine bonds. Older crystal ionic radii were based on the radius of (CN 6) equal to 119 pm these radii are 14-18 percent larger than the effective ionic radii. 

Pressure can also induce a change in the spin state of a transition-metal ion in a molecule or crystal with resultant change in the spectmm. The usual change observed is from high to low spin, but the inverse transition has been observed in some cases. 

Figure 19.16 The possible high-spin and low-spin configurations arising as a result of the imposition of an octahedral crystal field on a transition metal ion.

Fig. 4.20 A schematic view of the ground state terms of a transition metal ion with 3d electron configuration and spin S = 5/2, such as in high-spin iron(III), under various perturbations with decreasing interaction strength from left to right...

The branched tetradentate ligand tris(2-aminoethyl)amine (tren) forms rather stable metal complexes with most transition metal ions. It is a very hard and basic ligand and consequently its iron(II) complexes are all high-spin. Later we will discuss hexadentate derivatives of this ligand which form crossover complexes (see Sect. 3.2). 

If the coordination number of a given complex of a first row transition metal ion exceeds six there seems to be a general stabilization of the high-spin configuration. To our knowledge, there are no examples of Fe(II) crossover complexes with such high coordination numbers. 

Figure 9.29 Photo-induced spin crossover in transition metal ions (a) incident photons can successively promote electrons from the ground t2g state to the high-energy eg state and (b) spin crossover involving election transfer and excitation in KFeCo(CN)6-...

In the initial state, all transition-metal ions are in the low-spin state, giving an effectively nonmagnetic solid. Irradiation transfers an electron from Fe to Co and promotes electron excitation to create all high-spin ions with a considerable magnetic moment (Fig. 9.29b). The magnetic defects in this case are the triple group of metal ion-cyanide-metal ion. As in all materials for device use, variation in dopant concentrations can be used to tune the desirable properties of the solid. 

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