Nickel complexes planar isomers

The first Ni Mossbauer spectrum of nickel in a bioinorganic compound with determinable EFG and isomer shift was reported for a nickel complex compound with planar [NiSJ core and considered as a model compound for hydrogenase. This Mossbauer spectrum from the formal Ni compound is presented in Fig. 7.16. The observed quadrupolar interaction can be understood in terms of ligand field theory. In this approach, the b g and levels (d y2 and d ) are not occupied which is expected to cause a large negative EFG contribution [32]. 

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). 

The unstable czs-octahedral isomer of the nickel(II) complex of the macrocyclic ligand 13aneN4 (1,4,7,10-tetraazacyclotridecane) isomerizes to an intermediate planar isomer, which then converts to the stable planar isomer... 

At lower temperatures, separate signals for both cis and trans isomers can be recognized (129). The nickel complex 15 also exists as a mixture of cis and trans square-planar isomers (130), and these, too, interconvert rapidly in the NMR tube at higher temperatures. This reaction also is first order in complex and is unaffected by free ligand. 

The second series of related materials was based on the (3,4-didodecyloxyphenyl)diaminodi-oxime derivative (73). Vicinal dioximes are capable of coordinating through N,N or N,0 sites of the oxime groups. Upon the complexation, the ligands fold back, and the transition metal complexes are V,V-coordinated with a square-planar structure. Both isomeric forms were selectively isolated for the nickel complexes, but only the anii-isomer for the palladium complex. The two uwti-complexes complexes exhibited a Colh phase ((73) M = Ni Cr 78 Colh H7 I M = Pd Cr 80 Colh 131 I). Most astonishing, however, was the observation of a Colh phase for the isomeric amphi-mckel complex ( , Z-isomer), between 66 °C and 145 °C. Not only was the existence of mesomorphism surprising and unexpected, but the mesophase stability also increased with respect to the anti-complex. 

Unless retarded by repulsion between bulky substituents in the more crowded planar isomer, the tetrahedral-to-planar isomerization has a low enthalpy of activation, AH 10 4 kcal/mol [24]. Its entropy of activation is ordinarily quite negative for a unimolecular isomerization AS < —10 kcal/mol K) but is substantially less so - occasionally approaching zero - in nickel(II) complexes with halogen atoms as coordinating ligands [25]. This pattern of Arrhenius parameters, characteristic of reactions that occur with spin inversion (see Chapter 9), is hardly surprising in view of the fact that the tetrahedral complex is high-spin (5 = 1) whereas the square-planar complex is low-spin S = 0). 

These interconversions are quite rapid and have been studied by laser T-jump" and ultrasonic relaxation. Kinetic studies suggest that several nickel(II)-salicylaldimine complexes favor ligand substitution through the square-planar isomer. 

There is now a growing literature of nickel organometallic complexes that contain carbon dioxide or related cumulene ligands that result from reactions with carbon monoxide. The first structurally characterized complex of carbon dioxide was the nickel complex Ni(G02)(PCy3)2 reported in 1975. A more recent study of this complex provides the complete assignments of the vibrational spectra and theoretical calculations of different isomers in support of a mechanism for CO2 fluxionality that involves end-on coordination. The tridentate pincer ligand 2,6-bis((diiso-propylphosphino)methyl)phenyl (PGP) has been used to form the square-planar Ni(ii) hydroxide complex Ni(OH)(PGP). The complex Ni(OH)(PCP) reacts with CO to give a binuclear /X-GO2 complex (Equation (2)). 

Stereoisomeric mixture of nickel complexes of 16-membered macrocycle 32 was obtained using both templating and high-dilution procedures. The isomers of macrocycle 33 were liberated from nickel by cyanide (Scheme 12.14) [51]. Macrocyclic tetraphosphine demonstrated an ability to form square—planar complexes with various nickel salts [52]. 

Ligand (69) coordinates to nickel such that the four donors and the metal ion form a planar array whereas (68) coordinates around one face of an octahedral arrangement. Each complex type exhibits a characteristic kinetic inertness which no doubt arises from the operation of the macro-cyclic effect. Indeed, because of the inertness of the cation [Ni(tri)(H20)3]2+, its resolution into optical isomers has been possible... 

The unsymmetrical nature of / -mercaptoethylamine should lead to geometric isomerism among its metal complexes, cis and trans isomers might be expected with the square planar nickel (II) and palladium (II) derivatives and facial and peripheral isomers with cobalt (III). However, during the course of the preparation of various samples in which the procedure and experimental conditions were varied, no evidence of such isomerism was apparent (6, 15). This is particularly evident in the case of the cobalt (III) complex, CoL3. Samples prepared by the addition of cobalt (II) chloride 6-hydrate to strongly basic aqueous solution of the ligand and by displacement of ammonia and (ethylenedinitrilo)-... 

Ni(04C2)(tet-a)-3H20 (tet = 5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradecane a = meso isomer b = racemic isomer) were prepared by the reaction of a concentrated solution of sodium oxalate with an aqueous solution of the appropriate nickel(II) amine complex.1780,1781 In the dinuclear complex [Ni2(04C2)(en)4](N03)2 (234)1740-1741 the bridging oxalato group is planar and symmetrically bonded to the two nickel atoms. The same structure occurs in the complex Ni2(04C2)(0N0)2(py)6 which was obtained as a by-product in a very low yield when a pyridine solution of methanenitrosolic acid and nickel(II) were allowed to stand for several months.1741... 

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). 

The stereochemistry of the 3,10-meso- and 3i ,10i -isomers of 3,5,7,7,10,12,14,14-octamethyl-1,4,8,1 l-tetra-azacyclotetradeca-4,1 l-dienenickel(n) has been established by detailed study of their XH n.m.r. spectra.476 The X-ray structures of7,8,15,17,18,20-hexahydrodibenzo(e.m)pyrazino[2,3-b]-l,4,8,l l-tetra-azacyclotetradecinatonickel(u) (85) and the related complex (86) have been determined. Both contain four-co-ordinate nickel(n), there being some slight distortion from planarity 477... 

Formigue and coworkers have successfully prepared asymmetrically substituted dithiolenes such as edt-CN (edt-CN = 2-cyano-l,2-ethylenedithiolate), tfadt and their square-planar nickel (49 and 50) and gold complexes . Both cis and trails isomers coexist and interconvert in solutions, while in the solid state the trails form is more commonly... 

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