Octahedral complexes Spin equilibria

A variety of geometries have been established with Co(II). The interconversion of tetrahedral and octahedral species has been studied in nonaqueous solution (Sec. 7.2.4). The low spin, high spin equilibrium observed in a small number of cobalt(Il) complexes is rapidly attained (relaxation times < ns) (Sec. 7.3). The six-coordinated solvated cobalt(ll) species has been established in a number of solvents and kinetic parameters for solvent(S) exchange with Co(S)6 indicate an mechanism (Tables 4.1-4.4). The volumes of activation for Co " complexing with a variety of neutral ligands in aqueous solution are in the range h-4 to + 1 cm mol, reemphasizing an mechanism. 

S-Methyl-A-(2-pyridyl)methylenedithiocarbazate (nns) can be prepared by the condensation of S-methyldithiocarbazate and pyridine-2-aldehyde. The low-spin complexes [Fe(nns)2].X (X = CIO or FeCl ) have both been isolated.2,1.3-Benzothiodiazole. 2,1,3-benzoselenodiazole, and their derivatives (L) form the octahedral complexes FeL2Cl3. and morpholine-4-carbodithioate (mdtc) forms the complex [Felmdtclj]. Mossbauer and magnetic data for a series of monothio- -diketonate-iron(iii) complexes have been interpreted in terms of a thermal equilibrium between the sextet and doublet states. 

Table 10a High- and Low-spin Complexes in Octahedral Stereochemistry, Giving the Approximate Value of Dq/B for Spin Equilibrium...

The phenomenon of spin equilibrium in octahedral complexes was first reported by Cambi and co-workers in a series of papers between 1931 and 1933 describing magnetic properties of tris(iV,iV-dialkyldithio-carbamato)iron(III) complexes. By 1968 the concept of a thermal equilibrium between different spin states was sufficiently well established that the definitive review by Martin and White described the phenomenon in terms which have not been substantially altered subsequently (112). During the 1960s the planar-tetrahedral equilibria of nickel(II) complexes were thoroughly explored and the results were summarized in comprehensive reviews published by Holm and coworkers in 1966 and 1973 ( 79, 80). Also, in 1968, Busch and co-workers... 

The pressure dependence of the NMR spectrum of a nickel(II) complex which undergoes a coordination-spin equilibrium has been used to obtain the volume difference between the planar and octahedral isomers (118). In this case both the temperature and pressure dependence of the NMR spectra were analyzed simultaneously to yield five parameters, AH0, AS0, A V°, and the chemical shifts of the two isomers. Subsequent determinations from the electronic spectra and ultrasonics relaxation are in good agreement with the NMR result (13). 

The Raman laser temperature-jump technique has been used in studies of a variety of spin-equilibrium processes. It was used in the first experiment to measure the relaxation time of an octahedral spin-equilibrium complex in solution (14). Its applications include investigations of cobalt(II), iron(II), iron(III), and nickel(II) equilibria. 

The dynamics of an octahedral spin equilibrium in solution was first reported in 1973 for an iron(II) complex with the Raman laser temperature-jump technique (14). A relaxation time of 32 10 nsec was observed. Subsequently, further studies have been reported with the use of this technique, with ultrasonic relaxation, and with photoperturbation. Selected results are presented in Table III. 

The cobalt(II) complexes which undergo spin equilibrium are of several different types. Octahedral high-spin complexes with a T ground state are subject to Jahn-Teller distortion in the low-spin d1 2E state. This effect is best documented in structures of the Co(terpy)22+ spin-equilibrium complex. The high-spin isomer is nearly octahedral, with a difference in Co N bond lengths between the central and distal nitrogens of only 6 pm. In the Jahn-Teller distorted low-spin state this difference has increased to 21 pm (58). 

There are a few examples of spin equilibria with other metal ions which have not been mentioned above. In cobalt(III) chemistry there exist some paramagnetic planar complexes in equilibrium with the usual diamagnetic octahedral species (22). The equilibria are the converse of the diamagnetic-planar to paramagnetic-octahedral equilibria which occur with nickel(II). Their interconversions are also presumably adiabatic. Preliminary observations indicate relaxation times of tens of microseconds, consistent with slower ligand substitution on a metal ion in the higher (III) oxidation state (120). 

For the complex of Ni(II) and tetraazaundecane(L) there is an equilibrium between the yellow, unhydrated square-planar (low-spin) complex NiL and the blue octahedral (high-spin) form NiL(H2 0) ... 

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