Nickel complexes planar-tetrahedral equilibria

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. 

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... 

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. 

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). 

Acetylacetonates with bulky substitution on the 1 and 5 positions may form square planar complexes (22). Bis(2,2,6,6-tetramethyl-3,5-hep-tanedione)nickel(II) is square planar, whereas bis(2,6-dimethyl-3,5-hep-tanedione)nickel (II) exhibits an equilibrium between the square planar and tetrahedral configuration (22). 

Compounds of type c have been studied for cobalt(II), nickel(II), and copper(II) (48). Copper(II) complexes, with R smaller than <-butyl, are planar. Nickel (II) complexes, where R is sec-butyl, are involved in a planar pseudo-tetrahedral equilibrium. When R is smaller than sec-butyl, planar complexes are obtained. No planar complexes of cobalt(II) with this ligand were reported (48). 

Bis(pyrrole-2-aldiminato)nickel(II) complexes (125) are diamagnetic in the solid state when R = H, Pr, Pr and Et, and paramagnetic pseudotetrahedral when R = Bu. 1001-1003 In solution there exists an equilibrium between square planar and tetrahedral species when R = Pr1, Bus and Bu Such equilibria were also investigated for complexes of the type (126) obtained from the condensation reaction in basic media of o-aminobenzaldehyde and a number of diamines in the presence of nickel(II).1004-1007 Square planar complexes (127)1008,1009 and (128)1010 were obtained with deprotonated pyridinecarboxamide ligands. In these complexes the Ni—N (amide) bond distance (184-187 pm) is shorter than the Ni—N (pyridine) distance (192-195 pm). 

Four-coordinate d8 complexes can display a closely related electronic and geometric equilibrium between paramagnetic tetrahedral and diamagnetic planar isomers. Numerous examples are known in nickel(II) chemistry (80). In this case, as well as with the octahedral complexes described above, there is no change in the coordination number of the metal ion. 

Less expected, perhaps, are results on substitution reactions of four-coordinate nickel(II) chelate complexes which occur in equilibrium between planar and tetrahedral isomers. Despite the longer bond lengths of the tetrahedral isomers, it is the planar isomers which undergo the substitution reactions (140). 

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]. 

The following general type of complexes has been studied extensively (Figure 5). These bidentate ligands, which form bis-complexes of nickel (II), copper(II), and cobalt(II), are nonelectrolytes and therefore have moderate solubility in nonpolar, noncoordinating solvents. Furthermore, using these solvents reduces additional coordination. The equilibrium between the planar and pseudo-tetrahedral conformations may be altered by changes in solvent, temperature, and substituents. 

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