Mossbauer spectroscopy ligands

The application of Mossbauer spectroscopy in the study of mixed ligand complexes. K. Burger, Inorg. Chim. Acta, Rev., 1972, 6, 31-46 (125). 

DNA], with varying ligand organotin(IV) molar ratios, frozen at 77.3 K, were monitored by Sn Mossbauer spectroscopy. 

First isolated from D. desulfuricans (28), desulfoferrodoxin (Dfe) was also isolated from D. vulgaris (29). D is a 28-kDa homodimer that contains two monomeric iron centers per protein. These iron centers were extensively characterized by UV/visible, EPR, resonance Raman, and Mossbauer spectroscopies (30). The data obtained were consistent with the presence of one Dx-like center (center I) and another monomeric iron center with higher coordination number (penta or hexacoordinate), with 0/N ligands and one or two cysteine residues (center II). Comparison of known Dfx sequences led to the conclusion that only five cysteines were conserved, and that only one of them could be a ligand of center II (31). 

However, useful as it is, ligand field theory is not a predictive first principles theory. Thus, it cannot be used to predict a priori the Mossbauer parameters of a given compound. Yet, the need to do so arises fi equently in Mossbauer spectroscopy. For example, if a reaction intermediate or some other unstable chemical species has been characterized by freeze quench Mossbauer spectroscopy and its SH parameters become available, then the question arises as to the structure of the unstable species. Mossbauer spectroscopy in itself does not provide enough information to answer this question in a deductive way. However, the more modest question which structures are compatible with the observed Mossbauer parameters can be answered if one is able to reliably predict Mossbauer parameters... 

Since Mossbauer spectroscopy is sensitive to all terms in the SH, it is also sensitive to the ZFS and the g-tensor. The theory of both interactions can be approached along the same lines as explained in some detail in Appendix 1 (Part III, 3 of CD-ROM) [89, 90]. This becomes somewhat elaborate for the ZFS while the g-tensor is more readily approached. Both quantities have been previously treated in some detail and a protracted discussion would be inappropriate here [9, 79, 89-93]. In general, the accuracy with which both quantities can be calculated from DFT is rather moderate and a combination of ligand field theory and DFT or some... 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

Coordination compounds of dianionic dithiolene (S2C2 R2) and benzene-1,2-dithiolene (bdt = (S2C6H4) and their derivatives have been studied since the 1960s by Mossbauer spectroscopy [87] and other techniques. Nevertheless, many aspects of their electronic structure remained uncertain for a long time. The five-coordinate ferric complexes with two equatorial dithiolene ligands exhibit intermediate spin and show the Mossbauer parameters = 0.25-0.38 mm s and A q = 1.6-3.2 mm s For example, [Fe° mnt)2/ y] with two mnt ligands (=S2C2(CN)2) and an... 

Although in most of the above reports structural information is based on X-ray data, Au Mossbauer spectroscopy has also been successfully employed for the investigation of two-, three-, and four-coordination in gold(I) complexes.2551 An increase in coordination numbers leads to a decrease in the isomer shift (IS) by 1-2mm s 1 (three-coordination) or 2-4mm s 1 (four-coordination) relative to two-coordination. For the same ligands, the quadrupole splitting (QS) for three-coordinated complexes is expected to be very similar to that for the two-coordinate derivatives, while that for complexes with Td symmetry should be zero. 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

The results from Mossbauer spectroscopy in applied magnetic fields clearly prove that the spin transition in the dinuclear compounds under study proceeds via [HS-HS][HS-LS][LS-LS]. Simultaneous spin transition in both metal centres of the [HS-HS] pairs converting the dinuclear pairs directly to [LS-LS] pairs can apparently be excluded, at least in the present systems. This is quite surprising in view of the fact that the present dinuclear complexes are centrosymmetric (in other words the two metal centres have identical surroundings, and should therefore experience the same ligand field strength and, consequently, thermal spin transition should occur simultaneously in both centres). 

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