Planar, Tetrahedral

The lability inherent in the planar, tetrahedral equilibria which nearly all involve Ni(II) requires that nmr line broadeningor photochemical perturbation methods be used for their kinetic resolution. First-order interconversion rate constants for 

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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 large number of stable planar, tetrahedral, or octahedral complexes of lb, lib, and Vlllb elements (Cu2+, Zn2+, Co2+, Ni2+, Ru2+, Rh2+, Pd2+, Pt2+, Pt4+) using cyclopropenones (preferentially the diphenyl compound) as ligands has been evaluated mainly by Bird281. Their preparation may start either with metal salts or with carbonyls, as the octahedral Co(II) complex [Co(Dcp)6]2+ may exemplify ... 

Figure 4.1 shows the common structural types that describe a very large number of inorganic molecules. Linear, trigonal planar, tetrahedral, trigonal bipyramid, and octahedral structures result when... 

We consider the four structurally diverse classes of organometals I-IV, in which the configuration and coordination about the metal centers vary systematically from octahedral, square planar, tetrahedral to linear, respectively. 

Linear Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral... 

Diphosphinedihalonickel(II) complexes planar-tetrahedral spin equilibrium, 32 29-30 reaction coordinate profile, 32 31 Diphosphoric acid, dissociation constants for, 4 25... 

In solution the cfl complexes of Ni11 are characterized by several rapid equilibria between species having different coordination numbers and/or geometries. Upon irradiation, these equilibria are perturbed and can be studied by relaxation techniques. A number of photoinduced changes have been investigated in this manner including square planar/tetrahedral, four-coordinate/five-coordinate and four-coordinate/six-coordinate interconversions.138... 

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... 

For nickel(II) complexes involved in planar-tetrahedral equilibria, the difference in nickel(II)-ligand distances is only 5 pm. This relatively small difference is understandable when it is recognized that the t2 orbitals in tetrahedral complexes are only weakly a antibonding, in contrast with the strong a character of the eg orbitals in octahedral complexes. There is, of course, substantial rearrangement of bond angles. 

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 ... 

In this section the dynamics of spin equilibria of nickel(II) will be described, beginning with intramolecular planar tetrahedral equilibria and continuing with coordination-spin equilibria, in which bond formation and dissociation become involved. 

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. 

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