Nickel complexes planar-octahedral equilibria

Nickel complexes of type d have been studied by Sacconi (82-84,91) and similar compounds by Holm (16). If NR2 is an or /io-substituted anilino group, the complexes are planar and diamagnetic. If NR2 is anilino or para-substituted anilino, the substances are octahedral or planar, depending on X. In inert solvents, compounds with a substituted anilino group exist as an equilibrium mixture of octahedral and planar forms, with the planar form predominating at higher temperatures (83). If the NR2... 

In 1961, Cotton and Fadder 43) further found that Ni(acac)2 exists in a monomer-trimer equilibrium in diphenylmethane over a 80—200 °C range. At low temperatures the green octahedrally coordinated trimeric species predominates, but at 200 °C the monomeric, square planar nickel complex is stable giving a red solution. Other /9-diketonate-nickel(II) complexes found to participate in a monomeric-trimer equilibrium from spectral and magnetic studies were... 

Monomer-oligomer equilibria. [Ni(Me-sal)2], mentioned above as a typical planar complex, is a much studied compound. In pyridine it is converted to the octahedral bispyridine adduct (/zsoo = 3.1 BM), while in chloroform or benzene the value of is intermediate but increases with concentration. This is ascribed to an equilibrium between the diamagnetic monomer and a paramagnetic dimer, which must involve a coordination number of the nickel of at least 5 a similar explanation is acceptable also for the paramagnetism of the solid when heated above 180°C. The trimerization of Ni(acac)2 to attain octahedral coordination has already been referred to but it may also be noted that it is reported to be monomeric and planar in dilute chloroform solutions. 

SlOO proteins, calcium binding, 46 451-456 Spruhtrocken process, 4 26 Square-planar complexes, 4 157-164 octahedral, compared, 4 162-174 in solution, 34 270-271 Square-planar iridium complexes, 44 295, 297 Square-planar nickel macrocyclic complexes equilibrium with octahedral species, 44 116-118... 

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. 

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 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). 

The complexes Nin(L), where L = 3,10-di(/>-X-benzoyl)-2,4,9,11 -tetrame-thyl-l,5,8,12-monobenzotetraazcyclo[14]annulene, X = Me, H, Cl, N02, OMe, show H deshielding effects due to the benzoyl groups.401 The 111 and 13C NMR spectra of Ni(II) and Zn(II) complexes with the Schiff base from 1,2-bis(o-aminophcnoxy)cthanc and salicylaldehyde are consistent with 0,N,N,0-coordination of the ligand.402 The 3H NMR spectra of aqueous solutions of [Ni(L-0)]+ and [Ni(L-NH)(H20)3]2+, where L = flexidentate 5-substituted salicylaldimino Schiff bases based on l-(2-aminoethyl)piperazine, show that these exist as planar and octahedral forms in equilibrium.403 H NMR signals from the ortho-protons in bis[2-(2,4-dichlorophenylmethyleneamino)benzene-thiolato]nickel(II) are consistent with Ni. . . H-C interactions in solution.404... 

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