Ions, paramagnetic Subject

I now move on to paramagnetic metal complexes. Upadhyay et studied ruthenium(iii) compounds containing clusters of six ruthenium atoms. The Ru(iii) ion has an odd number of electrons which leads to paramagnetism. In the cluster, the spins of the six ions are subject to antiferromagnetic exchange interactions, which results in rather narrow NMR lines, the possibility to observe NOEs and measurably long... 

The chapter Electron Spin Resonance in Catalysis by Lunsford was prompted by the extensive activity in this field since the publication of an article on a similar subject in Volume 12 of this serial publication. This chapter is limited to paramagnetic species that are reasonably well defined by means of their spectra. It contains applications of ESR technique to the study of adsorbed atoms and molecules, and also to the evaluation of surface effects. The application of ESR to the determination of the state of transition metal ions in catalytic reactions is also discussed. 

At the beginning of this century slow magnetic relaxation in the paramagnetic phase of a ID system was reported for a Co2+ nitronyl-nitroxide chain [58]. The dynamics was under many respects very similar to that of SMMs and these ID systems were later named SCM to underline the analogies [9]. Since then an intense research activity, though not as spread out as for SMMs, has been devoted to SCMs. The interested reader can find extensive literature on the subject, including exhaustive reviews and a book chapter [59-63]. The aim here is to provide some basic concepts of the phenomenon and to introduce some selected examples of SCMs based on lanthanide ions. 

The tetrakis-dithiophosphinate complex [PPI14][Pr(S2PMe2)4], whose crystal structure has also been determined, has a distorted tetragonal antiprismatic geometry with Pr—S = 2.888-3.0150.400 The complex ions [M S2P(OEt)2 4]", have been the subject of an NMR paramagnetic shift study (see Section 39.2.9.4). 

The starting point for most of the redox chemistry considered in this review is the nickel(II) ion. The nickel(II) ion has a d8 electronic configuration and, with weak-field ligands such as H20, it forms a six-coordinate ion with approximately octahedral symmetry and a paramagnetic (two unpaired electrons) 3A2 ground state. The characteristic solution chemistry of six-coordinate nickel(II) is well documented and, in particular, the substitution behavior has been extensively studied and is the subject of recent reviews (11, 12). It is a labile ion with solvent exchange rates around 104 sec-1 at 25°C and activation parameters are consistent with dissociatively activated interchange behavior (13). 

Paramagnetic compounds can also be difficult NMR subjects because the unpaired electron(s) couple(s) with the nuclear spin to provide rapid relaxation. Relaxation times can, however, yield useful information about a system certain parameters which contribute to the magnitude of 7) are directly related to the oxidation number and spin state of the metal ion, the nature of the metal ion and its coordination geometry. For example, high-spin Com in octahedral complexes has a relaxation time more than an order of magnitude faster than the same ion in a tetrahedral arrangement of ligands (Banci et at., 1992). 

Although most lanthanide ions are paramagnetic, because of rapid relaxation effects, spectra can be obtained only at low temperatures (often 4.2 K) in most cases. From the point of view of the chemist, EPR spectra are readily obtained (at room temperature) only from the f Gd +, with its 87/2 ground state. The sublevels of this state are degenerate in the absence of a crystal field (in a free Gd + ion), but are split into four Kramers doublets, with M/-values of 1/2, 3/2, 5/2 and 7/2. The application of a magnetic field removes the degeneracy of each doublet, and transitions can occur on irradiation with microwave radiation, subject to the usual selection rule of AM/ = 1. 

Selected samples of anion exchange fabrics containing Oo anions or metallic ions capable of forming paramagnetic complexes with tertiary amine groups were subjected to EPR analyses. 

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