Cobalt 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. 

In both the Fe(II) and Fe(III) cases the spin state change involves a change in the population of the a antibonding eg orbitals of two d electrons. For the spin equilibrium of the d1 cobalt(II) complexes the population of the eg orbitals changes by only one electron. From examination of the structures of a series of [Co(terpy)2]2+ salts, a bond length difference between the two Co-N (central) distances of 21 pm was found between the spin states, with a difference of only 7 pm found between the four Co-N (distal) distances. This gives an average difference of 12 pm. 

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 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). 

Similar considerations apply to the role of spin equilibria in electron transfer reactions. For many years spin state restrictions were invoked to account for the slow electron exchange between diamagnetic, low-spin cobalt(III) and paramagnetic, high-spin cobalt(II) complexes. This explanation is now clearly incorrect. The rates of spin state interconversions are too rapid to be competitive with bimolecular encounters, except at the limit of diffusion-controlled reactions with molar concentrations of reagents. In other words, a spin equilibrium with a... 

The spin state lifetimes in solution of the complexes II and III have been measured directly with the laser Raman temperature-jump technique189). Changes in the absorbance at 560 nm (CT band maximum) following the T-jump perturbation indicate that the relaxation back to equilibrium occurs by a first-order process. The spin-state lifetimes are r(LS) = 2.5 10 6 s and r(HS) =1.3 10 7 s. The enthalpy change is AH < 5 kcal mol-1, in good agreement with that derived from x(T) data in Ref. 188. The dynamics of intersystem crossing processes in solution for these hexadentate complexes and other six-coordinate ds, d6, and d7 spin-equilibrium complexes of iron(III), iron(II), and cobalt(II) has been discussed by Sutin and Wilson et al.u°). 

Nickel(ii) and cobalt(ii) complexes continue to be the most widely studied first-series transition metal complexes. The well resolved NMR spectra arise from the very rapid electron-spin relaxation which occurs as a result of modulation of the zero-field splitting of these ions. In the case of 4-coordinate nickel(ii), only tetrahedral complexes (ground state Ti) are of interest since the square-planar complexes are invariably diamagnetic. Many complexes, however, undergo a square-planar-tetrahedral dynamic equilibrium which can be studied by standard band-shape fitting methods (Section B.l). 

In special circumstances the metal and a ligand can compete for the spin in a paramagnetic complex. As this alternative involves oxidation-state changes, it is referred to as valence tautomerization or redox isomerization [78]. Such behavior is observed for o-semiquinone complexes of cobalt and manganese [78] recently, a copper(I)-semiquinone-copper(II)-catecholate equilibrium system (7) of biochemical interest has been analyzed by temperature-dependent ESR [79]. 

The low-spin cobalt(III) dimethylglyoxime parent complex is inert even in the presence of only a small excess of dimethylglyoxime it is formed quantitatively, and the dimethylglyoxime cannot be displaced from the complex even by a comparatively large excess of monodentate ligand. At the same time, the displacement of the coordinated solvent molecules by appropriate monodentate ligands is a reversible process leading to equilibrium within a relatively short time. 

EPR spectroscopy was employed by Rockenbauer et al [Ro 72] to study the equilibrium stability constants of low-spin cobalt(II) mixed complexes. They determined the equilibrium constants of formation of the mixed complexes of the bis(dimethylglyoximato)cobalt(II) parent complex with pyridine ligands, in methanol. It was shown that in a methanolic solution of the parent complex, two methanol molecules are coordinated along the z axis, and these methanol molecules can be replaced stepwise with pyridine. 

Reactions of amminecobalt(m) complexes as oxidants are being further investigated. Under pseudo-first-order conditions, the two reactions observed in the reduction of [Co(NH3)4(OH)2] + by cysteine have been ascribed to the faster redox involving the traizj-isomer and a slower cis reaction. The order with respect to complex is unity and that for cysteine is zero. The mechanism proposed involves the rapid pre-equilibrium to form [(RS)Co(NH3)4(OH2)] + complexes which then undergo a rate-determining internal redox process. More data are required for these systems which appear to involve a pre-equilibrium rate much greater than expected for a low-spin [Pg.88]

The kinetics of the binding of anions and sulphonamides to carbonic anhydrase have been measured by the stopped-flow technique. The results are consistent with a mechanism involving a pH-dependent equilibrium between two co-ordination forms of the enzyme in which anions selectively combine with the low-pH form of the enzyme whereas sulphonamides combine with the high-pH form. The effect of pH on the anion affinity correlates with the pH dependence of the spectral change associated with the cobalt(n) form of the enzyme. Further evidence on the similarity of the conformations at the active sites of the zinc and cobalt(n) forms of carbonic anhydrase has been provided by spin-labelling. A nitroxide-substituted sulphonamide was used as the spin label and its e.s.r. spectrum was found to be almost identical in the two forms of the enzyme-inhibitor complex. 

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