Transition metals paramagnetism

Representative Organo Transition metallic Paramagnetic Species... 

Transition metal paramagnetic complexes that warrant further comment are the radical anions derived from metallocene- (especially ferrocene-) ketyls and related species, such as compounds IX (61), X (45), or XI (146), and metallonitroxides. From the ESR spectra of compounds... 

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

The simplest of the ir-bondcd Re-C compounds is the green, paramagnetic, crystalline, therm ly unstable ReMen, w ich, after WMe, was only the second hexamethyl transition metal compound to be synthe zed 11976). It reacts with LiMe to give the unstable, pyrophoric, Lii[ReMe(,, which has a square-antiprismatic structure, and incorporation of oxygen into the coordination sphere greatly H reases the stability, wit e,ss Re CMe, which is thermally stable up to 200 C, and Re "0[Pg.1068]

The angular overlap model for the description of the paramagnetic properties of transition metal complexes. A. Benici, C. Benelli and D. Gatteschi, Coord. Chem. Rev., 1984, 60,131 (204). 

However, while transition-metal ions often contain unpaired electrons, they exhibit none of the reactivity that is commonly associated with such radicals outside the d block. There is no behaviour comparable to that of the highly reactive and short lived radicals such as CH3. Also associated with the presence of unpaired electrons in these species is the phenomenon of paramagnetism. The long-term stability of many compounds with unpaired electrons is a characteristic of the transition-metal series. 

Derived from the German word meaning devil s copper, nickel is found predominantly in two isotopic forms, Ni (68% natural abundance) and Ni (26%). Ni exists in four oxidation states, 0, I, II, III, and IV. Ni(II), which is the most common oxidation state, has an ionic radius of —65 pm in the four-coordinate state and —80 pm in the octahedral low-spin state. The Ni(II) aqua cation exhibits a pAa of 9.9. It forms tight complexes with histidine (log Af = 15.9) and, among the first-row transition metals, is second only to Cu(II) in its ability to complex with acidic amino acids (log K( = 6-7 (7). Although Ni(II) is most common, the paramagnetic Ni(I) and Ni(III) states are also attainable. Ni(I), a (P metal, can exist only in the S = state, whereas Ni(lll), a cT ion, can be either S = or S =. ... 

For a reUable extraction of distances, it is important that dipolar relaxation is strongly dominating other relaxation processes. Hence, it is important to avoid paramagnetic ions or molecules such as transition metals or (paramagnetic) oxygen. Especially solution of small molecules therefore have to be carefully degased. 

The trace of D vanishes when dipole coupling between paramagnetic centers determines the ZFS, since dipole interaction is traceless. A typical example is the ZFS of triplets arising from coupled radical pairs, for which SOC is negligible. For transition metal ions in contrast, SOC is the leading contribution to ZFS and the trace of Zl in general has finite values. 

F.E. Mabbs and D. Collison, Electron Paramagnetic Resonance of d Transition Metal Compounds, Elsevier, Amsterdam, 1992. 

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