Magnetostructural Correlations

4 Consider the complex sketched in this box and predict the nature of the coupling when site A is occupied by Ni + and site B by Cr +. The out of plane TM-ligand distances are larger than the in-plane distances. 

What is the number of exchange paths when Cr + is replaced by Mn + What coupling can be expeced  

According to the HTH model this gives a reduction of the antiferromagnetic contribution to the magnetic coupling (see Eq.4.21) and illustrates the anti-synergistic effect or counter-complementarity of the two ligands. 

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For magnetostructural correlation, intimate knowledge of the molecular structure is a prerequisite, and hence the following discussion is mainly restricted to compounds characterized by X-ray crystallography. /-values are given with respect to the H= —J,jS,Sj Hamiltonian (reported values have been adapted for cases where the H= —2J S,Sj Hamiltonian has been used in the literature). 

As a part of their efforts to model dinuclear copper active sites, Reed and co-workers reported, using alkoxo-based dinucleating ligands, a few very interesting systems from the viewpoint of magnetostructural correlations (379) (TBP Cu-Cu 3.325A 2.1 278cm ), (380) (TBP ... 

Christou, Flendrickson, and co-workers reported triply bridged, ferromagnetically coupled dicopper(II) complexes (630) and (631).504-506 Perlepes et al. reported alkoxo-/carboxylato-bridged complexes (632) and (633).506 Using hydroxo-/carboxylato-bridged dicopper(II) complexes, Chakravarty and co-workers investigated magnetostructural correlations complexes (634) and (635)... 

B22 G (0, l/-y/6) in the Wybourne s notation [19, 32]. If literature-reported or experimentally determined parameters do not conform to this convention, rotation of the reference system should be applied, resulting in a standardized form of CF parameters [33]. This is of fundamental importance if different sets of parameters are to be compared to derive magnetostructural correlations and the direction of the quantization axis, and thus of the principal anisotropy axis, appropriately defined. 

This chapter is a comprehensive compilation of the fast accumulating hterature on discrete cyanide complexes, as opposed to extended structures of the PB type (Fig. 1) and other cyanide materials that were covered in previous reviews (17-19). We chose to hmit the scope of the current work to the molecular compounds, taking into account their fundamental importance from the molecular magnetism viewpoint. The finite structure of such complexes and their well-defined metal coordination environments allow for the establishment of magnetostructural correlations and the development of theoretical models that can be used as a starting point for the analysis of extended magnetic stmctures. 

Binary pyrazolate complexes containing divalent metals bridged by two pz groups are known for many transition metals. The increasing interest for this class of complexes stems from the expectation that they may provide useful insights in the field of magnetostructural correlations as well as in multimetal centered catalysis. 

Figure 10. Drawing of the dinucleating ligands utilized in the magnetostructural correlations on copper(ll) pyrazolato complexes.

Boyd and Martin (229) summarized magnetostructural correlations in [Ni4(ju,3-OR)4] and [Ni4(/u.3-OH)4] + cubanes in a detailed study of susceptibility and magnetization on [Ni4(0H)4(tach)4](N03)4 7H2O. The analogous Cr complex of this cyclohexyl-triamine (tach) ligand. 

Finally, the study of a range of star-shaped Fe4 clusters has actually allowed devising a magnetostructural correlation for second order anisotropy terms [Fe4(L)2 (dpm)g] (Hdpm = 2,2,6,6-tetramethylheptane-3,5-dione H3L = R 0CH2C(CH20H)3 with R = alkyl and related moieties) [58, 59]. A clear... 

Other Gd -3d compounds are less documented and characterized, and have no strong tendency to be ferromagnetic or antiferromagnetic like the obvious magnetostructural correlation of Gd -Cu systems. It seems that the magnetic coupling properties are mainly governed by the nature of the chemical linker between the spin carriers, but this is... 

Some other examples have been also reported for Ln -3d and Ln -radical systems. But there is no general magnetostructural correlation deduced from the reported compounds owing to the limited number of studies. These studies are still restricted by the difficulties of theoretical modeling and calculation for electronic structure of rare earths. 

M. Ruben, E. Breuning, J.M. Lehn, V. Ksenofontov, F. Renz, P. Giitlich, G.B.M. Vaughan, Supramolecular Spintronic Devices Spin transitions and magnetostructural correlations in [Fe4L4] [2 X 2]-grid-type complexes. Chem. Eur. J. 9, 4422-4429 (2003)... 

Romero FM, Ziessel R, Bonnet M, Pontillon Y, Ressouche E, Schweitzer J, Delley B, Grand A, Paulsen C (2000) Evidence for transmission of ferromagnetic interactions through hydrogen bonds in alkyne-substituted nitroxide radicals magnetostructural correlations and polarized neutron diffraction studies. J Am Chem Soc 122 1298-1309... 

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