Cobalt complexes magnetic properties

Some efforts have been made to interpret the spectroscopic and magnetic properties of cobalt enzymes in terms of coordination geometry and chemical identity of ligands. The basis of these attempts is a comparison with the corresponding properties of low-molecular weight complexes of known structure. A brief summary of relevant data on some models is given in the following section. 

Goodgame, M. Cotton, F. A. (1961) Magnetic investigation of spin-fiee cobaltous complexes IV. Magnetic properties and spectrum of cobalt(II) orthosilicate. J. Phys. Chem. Solids, 65, 791-2. 

The theory for the interpretation of the spectral properties of cobalt(II) is essentially known6-9, but no extensive compilation of experimental data is to our knowledge available. We want to fill this gap in the literature giving a comprehensive review of the ligand field interpretation of the electronic structure of high spin cobalt(II) complexes, as well as of the most common spectral and magnetic techniques which are used to characterize cobalt complexes. 

Every attempt to understand the spectral and magnetic properties of cobalt(II) complexes must start from the ligand field energy levels. Their dependence on the ligand field... 

Many fewer high-nuclearity cages are known for cobalt than for manganese or iron, and their magnetic properties have not been well studied. Unless otherwise stated, magnetic data have not been reported for the following complexes. 

Although the focus of this section has primarily been on iron and copper complexes, probably the most important transition metals biologically studied by the MCD technique, variable temperature and field dependence studies have also been carried out for complexes of other transition metals such as cobalt and manganese and the techniques described for iron and copper can easily be applied to other metals based on the nature of the ground state. MCD spectroscopy has the key advantage, over other techniques used to study bulk magnetic properties of an entire sample, that spectral bands associated with specific mefal cenfers can be sfudied in isolation. 

Russian workers have reported some terpyridyl complexes of co-balt(III) 35), and the bis(terpyridyl) cobalt(II) ion is now known. The magnetic properties of Co(terpy) depend critically on the anion present in the crystal thus, at 20°C the magnetic moments are 4.3 (perchlorate), 2.7 (bromide dihydrate), 2.1 (chloride monohydrate), and 2.2 B. M. (aqueous solution). A study of the temperature dependence of the moment for Co(terpy)2Br2 2H20 indicated that no simple explanation was possible 379). The subject has been considered more recently by Judge and Baker 412a). Some peculiar bipyridyl derivatives of cobalt(II) cyanide were reported some years ago 571) these could warrant further investigation. The UV spectra of Co(II) and Co(III) complexes have also been measured. 

The formation of a covalent complex in which the cobalt possesses three unpaired electrons and is covalently bound to six ammonia molecules can only occur by the excitation of two of the electrons to the 55 and 5 orbitals, which would require considerable energy. On the basis of the magnetic properties of the complexes of tetrammino cobalt, Pauling has described [Go(NH3)g] + as a predominantly covalent complex and [Go(NH3)g] + as predominantly ionic. In confirmation of this conclusion we find that Go(NH3)3013 is not stable and dissociates in water yielding ammonium ions, whereas the Go(NH3)3Gl3 complex is quite stable and the ion [Go(NH3)3] + does not decompose in solution. The Go—distance in [Go(NH3)3] + is 1 9 A, considerably shorter than that in [Go(NH3)q]2+ which is 2 5 A. 

Since aluminium has the electron configuration which becomes when trivalent, this compound will be diamagnetic. The analogous structure for iron, will leave five electrons unpaired in the d orbitals in agreement with the magnetic properties. In cobalt, d s y it must be assumed that the bonds are formed by pairing of the three unpaired electrons with the formation of a diamagnetic complex. 

The extremely insoluble, oxygen- and moisture-insensitive complex obeys the Curie law between 85 and 300°K, with a constant effective magnetic moment of 5.80 BM. The magnetic properties of the nickel, cobalt, and copper analogues are given elsewhere. ... 

The chromium(III) complexes represent the most simple systems in which magnetic interactions between paramagnetic ions can be studied. The cobalt(lll) systems may serve as matrixes in which the intermolecular interactions between the paramagnetic ions can be diminished. In these connections it is noteworthy that a mixed dihydroxo complex, (-)s89-A,A-[(en)2Cr(OH)2Co(en)2]has been described recently. This compound may serve as a model compound that represents the magnetic properties of the individual chromium(III) ion of the di-/a-hydroxochromium(lII) complex in the absence of a pair interaction. 

In complex 23, each copper atom is roughly square planar, with apical contacts involving some of the cartoonylic oxygen atoms of the dcmpz ligands for the external copper(II) centers. The synthesis, characterization, and magnetic properties of a series of Cobalt(II) complexes containing the pyrazolate anion... 

---