Axial ground state

The effect of axial ligands on ground state properties of complexes with orbitally degenerate ground terms. G. A. Webb, Coord. Chem. Rev., 1969, 4,107-145 (151). 

Fig. 9. Zero-field splitting of the S = 2 ground state of [Fe3S4] clusters. Constructed for D = -2.5 and E/D = 0.23, where D and E are the axial and rhombic zero-field splitting parameters, respectively.

The characteristic derivative-shaped feature at g 1.94 first observed in mitochondrial membranes has long been considered as the sole EPR fingerprint of iron-sulfur centers. The EPR spectrum exhibited by [4Fe-4S] centers generally reflects a ground state with S = I and is characterized by g values and a spectral shape similar to those displayed by [2Fe-2S] centers (Fig. 6c). Proteins containing [4Fe-4S] centers, which are sometimes called HIPIP, essentially act as electron carriers in the photoinduced cyclic electron transfer of purple bacteria (106), although they have also been discovered in nonphotosynthetic bacteria (107). Their EPR spectrum exhibits an axial shape that varies little from one protein to another with g// 2.11-2.14 and gi 2.03-2.04 (106-108), plus extra features indicative of some heterogeneous characteristics (Pig. 6d). 

The first step in the solution of equation (10.28b) is to hold the two nuclei fixed in space, so that the operator drops out. Equation (10.28b) then takes the form of (10.6). Since the diatomic molecule has axial symmetry, the eigenfunctions and eigenvalues of He in equation (10.6) depend only on the fixed value R of the intemuclear distance, so that we may write them as tpKiy, K) and Sk(R). If equation (10.6) is solved repeatedly to obtain the ground-state energy eo(K) for many values of the parameter R, then a curve of the general form... 

Cyanide binds to a series of Ni complexes with N302 chelate ligands, and EPR spectra of the adducts indicate quasi-axial symmetry with a g pattern typical of low-spin, six-coordinate complexes with axial elongation and with a 2Ay ground state. No direct spin interaction of the unpaired electron with the carbon atom of the cyanide takes place.70... 

Complex (77) has been obtained by electrolysis of the respective Ni11 species (i 1/2= —0.78 V vs. Cp2Fe/Cp2Fe+ in DMF) and represents the first Ni111 complex with aliphatic thiolato donors that could be characterized crystallographically.300 The overall square planar structure is retained, but the Ni—N and Ni—S distances shorten by 0.023 A and 0.057 A, respectively, upon oxidation. While (77) is EPR-silent, its pyridine adduct shows an axial spectrum (g = 2.313, 2.281, 2.000) with N-hyperfine coupling of the z-component, indicative of a (r )1 ground state. 

For d8 systems in pseudo-axial symmetry Figure 11 clearly shows that a 32 (a2 rr2 4) ground state is to be expected for all DtlB values, and this is confirmed by the observation... 

However, in pseudo-axial symmetry, all three ground states considered will be split by the combined action of the ligand field and spin-orbit coupling thus 32 -> 2+ + II,... 

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