Spin-sextet

In a simple crystal-field description, a spin-quartet ground state (S = 3/2) of iron(III) is energetically stabilized over the spin-sextet (S = 5/2) when the energy separation... 

Fig. 2.20. ZFS of the spin sextet levels of high spin iron(III).

Since it can be predicted that depopulation of the (high-spin) sextet level in favour of the (low-spin) doublet level should be accompanied by a contraction of the [FeS6] core in spin transition systems, it is to be expected that the position of the spin equilibrium K will depend on pressure. This has been experimentally verified for several tris(dithiocarbamato)iron(III) complexes ita chloroform solution at pressures up to 5000 atmospheres at room temperature. On the assumption that the molecular volume contraction AV arises entirely as a result of the spin change, equation (15) can be applied to the calculation of the volume change. Values of A V of the order of 5 cm3 mol-1 were obtained for systems with a wide variety of N substituents.289,290 The values correspond to a contraction of the Fe—S bond length of about 0.1 A, during the 5 A - 2T transition. 

Proceeding to nitrogen as the ligator atom, one usually observes a spin-flip from t2g eg to t2g, though thorough studies are scarce [42], small spin sextet-to-doublet separation energies can be suggested (vide infra). We refrain to go into details, because the cross-over is only weakly influenced by JT forces. 

In the weak field case, Flund s rule holds, and the state with the highest spin is the ground state (high spin). In the case of Mn + (d ), A is small for the octahedral Mn(OFl2) complex, and therefore the five d electrons have the same spin direction (2S -I-1 = 6 thus a spin sextet). 

The amount of spin contamination is given by the expectation value of die operator, (S ). The theoretical value for a pure spin state is S S + 1), i.e. 0 for a singlet (Sz = 0), 0.75 for a doublet (S = 1/2), 2.00 for a triplet (S = 1) etc. A UHF singlet wave function will contain some amounts of triplet, quintet etc. states, increasing the (S ) value from its theoretical value of zero for a pure spin state. Similarly, a UHF doublet wave function will contain some amounts of quartet, sextet etc. states. Usually the contribution from the next higher spin state from the desired is... 

S is the spin quantum number. The expected magnetic moments for a sextet and a doublet state are 5.91 and 1.73 respectively, measured in Bohr magnetons. The values calculated from the observed paramagnetic susceptibilities of the crystals are 5.88 for (NH aFeFg and 2.0 for... 

Let us now turn to the description of the reaction pathways. Figure 13-5 schematically depicts the shapes of the corresponding potential energy curves for the sextet and quartet spin-states and Table 13-11 contains the thermochemical information obtained at different levels of theory. 

An electron-gain centre of similar geometry and electronic structure is generated (43) by radiolysis of the nitrosocarbonyl Mn(C0)4N0. Spectra associated with the electron-loss centres Mn(C0)nX+ (n=4 or 5) are less well-defined and pose analytical difficulties (41). However, there is little doubt that these are high-spin radicals, probably electronic sextets. 

In Ag-SAPO-ll/C2H4 zeolite the EPR at 77 K shows the spectra of Ag° atoms and C2H5 radicals. After annealing at 230 K those species disappeared and then an anisotropic EPR sextet was recorded. Based on DFT calculation the structure of complex was proposed in which two C2H4 ligands adopted eclipsed confirmation on either side of the Ag atom. As a result the overwhelming spin density is localised on ethylene orbitals. 

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