Nickel complexes 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). 

There have been very few reports of the Raman spectra of spin-equilibrium complexes. In one experiment the presence of both high-spin and low-spin isomers of an iron(II) Schiff base complex was observed by the resonance Raman spectra of the imine region (11). The temperature dependence of the spectra was recorded for both solid and solution samples. Recently differences were described in the resonance Raman spectra of four- and six-coordinate nickel(II) porphyrin complexes which undergo coordination-spin equilibria. These studies are extensions of a considerable literature on spin state effects on the Raman spectra of iron porphyrins and hemes. There are apparently no reports of attempts to use time-resolved Raman spectra for dynamics experiments. 

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. 

It is possible to perturb a spin equilibrium by photoexciting one of the isomers. Among the possible radiative and nonradiative fates of the excited state is intersystem crossing to the manifold of the other spin state. Internal conversion within this manifold ultimately results in the nonequilibrium population of the ground state. If these processes are rapid compared with the relaxation time of the spin equilibrium, then the dynamics of the ground state spin equilibrium can be observed. This experiment was first performed for spin equilibria with a coordination-spin equilibrium of a nickel(II) complex (85). More recently a similar phenomenon has been observed in the solid state at low temperatures (41). The nonequilibrium distribution can be trapped for long periods at... 

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). 

Like [Ni(cyclam)], the above dinuclear nickel complex exists in aqueous solution as a mixture of low-spin, four-coordinated and high-spin, six-coordinated species, with the equilibrium between the two being dependent on the ionic strength. In the presence of excess per-... 

Nickel(II) complexes of (505) exhibit spin equilibria in solution.1355 With the bidentate analogues (506), complexes [Ni(506)2] have been isolated.1356 When Rj = Ph, the complex is tetrahedral in solution. It has a temperature independent magnetic moment of 2.75pB- When R = Me, the complex exhibits square planar-tetrahedral equilibrium in solution. Both are, however, diamagnetic in the solid state. 

A change in the spin state of a metal ion also can accompany a change in coordination number. Again, in some cases conditions may be established in which an equilibrium exists between two complexes with different coordination numbers and different numbers of unpaired electrons. Some of the concepts which are used to describe intramolecular spin equilibria can be extended to the description of these coordination-spin equilibria. Examples include equilibria among four-, five-, and six-coordinate nickel(II) complexes and equilibria involving coordination number changes in iron porphyrin complexes and in heme proteins. 

NMR has not proved generally useful, however, for examining the dynamics of spin equilibria. Low-temperature proton NMR has been used successfully to obtain rates for some planar-tetrahedral equilibria in nickel(II) complexes (99, 129, 130, 134). Equation (1) illustrates the orbital occupancy and ground state terms for the d6 equilibrium ... 

Planar-tetrahedral equilibria of nickel(II) complexes were the first spin-equilibria for which dynamics were measured in solution. It had been known that such complexes were in relatively rapid equilibrium in solution at room temperature, for their proton NMR spectra were exchange averaged, rather than a superposition of the spectra of the diamagnetic and paramagnetic species. At low temperatures, however, for certain dihalodiphosphine complexes, it is possible to slow the exchange and observe separate resonances for the two species. On warming the lines broaden and coalesce and kinetics parameters can be obtained. Two research groups reported such results almost simultaneously in 1970 (99,129). Their results and others reported subsequently are summarized in Table V. 

Historically, bis(aminotroponeiminato) nickel(II) complexes have been veiy instructive. The compounds are either pseudotetrahedral or display a tetrahedral-planar equilibrium. The ligands contain seven-membered rings showing alternation of proton shifts and spin densities (Table 2.5). The interest lies in the variety of R derivatives which show how spin density can be transmitted through it bonds, whereas it cannot be transmitted through sp3 carbons or through ethereal oxygen atoms [48,49]. 

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). 

The complexes with nickel(II), palladium(II), platinum(II), zinc(Il), cad-mium(II), mercury(II), and lead(II) and their adducts with nitrogen heterocycles have been described. The magnetic moment of the iron(III) complex is temperature-dependent owing to a spin-paired spin-free equilibrium. ... 

Nucleophilic reactions of the spin-paired tris(o-phenanthroline) iron(II) ion are bimolecular 70-72). The tris complex is close to the spin-free complex in energy since dithiocyanatobis(o-phenanthroline) iron(II) exists in a spin-free = spin-paired equilibrium 53). The corresponding tris(o-phenanthroline)nickel(II) ion is unaffected by the same nucleophile, which probably rules out Sat2 attack on the organic ring as the predominant factor. 

In aqueous solution, the complexes of most metal cations exist in dynamic equilibrium with their components. If we disturb this equilibrium, another one is instantly formed. It is quite otherwise with robust complexes which persist for hours (or even days) under conditions favourable to their decomposition any biological properties that they may have are strikingly different from those of their components. Robust complexes are formed where metal ions have 3,4 (low spin), 5, or 6 d electrons provided that formation of the complex involves large values of ligand-field stabilization energy. Metals most prone to form robust complexes are the transition metals platinum, iridium, osmium, palladium, rhodium, ruthenium, also (but not so frequently) nickel, cobalt, and iron. The halide and, particularly, the cyanide anions most readily form robust complexes with these transi-... 

---