Nickel planar/tetrahedral

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... 

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. 

The available evidence thus suggests that relaxation times for planar-tetrahedral equilibria in nickel(II) complexes in solution at room temperature fall in the range 0.1-10 /isec, corresponding to rate constants of the order 105-107 sec-1. These relaxation times are several orders of magnitude longer than those observed for octahedral spin equilibria. The reaction coordinate for the planar-tetrahedral equilibria is characterized by large enthalpies of activation for the reaction in both directions, in contrast with a relatively low enthalpy of activation for the high-spin to low-spin process in octahedral iron complexes. 

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

Among 4-coordinate transition metal complexes fluxional behavior based on planar/tetrahedral interconversions is of considerable importance. This is especially true of nickel(II) complexes, where planar complexes of the type Ni(R3P)2X2 have been shown to undergo planar tetrahedral rearrangements with activation energies of about 45 kJ mol 1 and rates of —105 s 1 at about room temperature. 

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). 

This is particularly true now with the recent discovery of trigonal prismatic structures for some 6-coordinated metal complexes (21). Surely the proper metal-ligand combination should give a system in which the difference in energy between an octahedral and a trigonal prismatic structure is small. As a result, racemization may take place by a trigonal twist mechanism. Twisting processes of the square-planar-tetrahedral type are known to occur readily in certain nickel(II) complexes (20). 

Numerous aminotroponeimineate complexes of type e, with nickel (II), have been prepared and are found to undergo square planar-tetrahedral interconversion (27). Extensive nuclear magnetic resonance data have been accumulated on this system along with magnetic moment and electronic spectral measurements (27). 

Figure 3.5 Simplified nickel complexes used in this lab. All contain two ammonia ligands and two hydroxyl ligands. Shown in this figure are the geometries used in this lab square planar, tetrahedral, bipyramidal, and square pyramidal, respectively.

Bond orbitals and stereochemistry. The number and direction of the valence bonds in compounds depends on the particular electron orbitals which form the bonds. The principles may be seen by referring to 6-coordinated cobalt, planar 4-coordinated nickel, and, tetrahedral 4-coordinated... 

These generalizations hold good for normal covalent bonds as well as coordinate bonds, for there is no fimda-mental difference between the two. It is interesting to note that Ni(C0)4, containing 4-covalent nickel, is tetrahedral, not planar, because the 3d orbitals are all filled and the bonding is done through one 4s and three 4p orbitals. 

King has recently studied the first examples of chelating tridentate phosphine-phosphite ligands, i.e., i P[CH2CH2P(OCH3)2]2 (R = CH3 and These ligands react with metal(II) chlorides of Fe, Co, and Ni in methanol solution to form the [MC1(P3 ligand)] cations, which are planar for nickel and tetrahedral for cobalt and iron. ... 

THE SQUARE PLANAR-TETRAHEDRAL ML4 INTERCONVERSION 307 TABLE 16.1. Mean Nickel-Ligand Distances (A) ... 

The kinetics of planar tetrahedral isomerization at nickel have been the subject of a theoretical disquisition. ... 

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