Nickel planar/octahedral

Square-planar complexes of platinum(II) and palladium(II) have been known for a long time the comparatively simple unit cells of compounds such as K2PdCl4, K2PtCl4, and Pd(NH3)4Cl2H20 led to early elucidation of the structures (257) and they all contain square-planar ions. The simple halides PdCl2 and Pt,Cl2 (71) consist of chains in which the metal is bonded from the corners of a square. Nickel chloride, on the other hand, has a layer lattice in which the nickel is octahedrally coordinated, and in the halide complexes the coordination is tetrahedral, as described in Section IV,B. 

The properties of nickel(II) complexes with unsaturated macrocycles which contain pyridyl groups are included in a very comprehensive review article.2626 The complexes are usually square planar with the exceptions of the trans octahedral NiX2(CR)2743 (CR = 2,12-dimethyl-3,7,ll,17-tetraazabicyclo[11.3.1]heptadeca-l(17),2,ll,13,l5-pentaene) and of the diamagnetic square pyramidal NiBr(CR)]Br-H20 (381).2747 The diamagnetic complexes Ni(CR)(C104)2 give rise to square planar octahedral equilibria in coordinating solvents,2744,2746 whereas... 

In the planar-octahedral equilibria of nickel(II) the d orbital population changes by transfer of one electron from the d2 orbital to the dx2-y2 a antibonding orbital. This results in a substantial increase in the nickel-nitrogen distances in the plane. Accompanying this is the formation of new metal-ligand bonds in the axial positions. 

Much of the focus of these studies has been on the relation between ligand substitution reaction mechanisms on octahedral nickel(II) and the dynamics of the planar-octahedral equilibria. For typical octa-... 

Fig. 7. Alternative reaction coordinate profiles for planar-octahedral equilibria of nickel(II).

Some diamagnetic planar nickel(II) complexes add only one ligand to form paramagnetic five-coordinate species. The dynamics of several of these equilibria have been examined by photoperturbation or NMR methods. The rate constants present in Table VII are of the order 106 sec 1 for the dissociation of the ligand from the five-coordinate species. These rates are comparable with those of the planar-octahedral equilibria and are consistent with the mechanistic interpretation presented above. 

The [H4(65b)]2+ gives dinuclear nickel complexes, where the macrocycle is essentially planar. Each nickel is octahedral. When the reaction is carried out using nickel acetate, a tetranuclear... 

A compound quite similar to the last one discussed in Sect. 6.2 has been investigated in a thorough manner and has revealed quite a new aspect of the complementarity between the susceptibility and EPR techniques89. Its formula is Cu(salen)Ni(hfa)2 and the structure of the heterobimetallic unit is shown in Fig. 19. As in the compound shown in Fig. 15, the copper is in planar and the nickel in octahedral surroundings, but the... 

Three relaxation processes observed during the Q-switched laser photolysis of dibromo-[l,3-bis(diphenylphosphino)propane]nickel(ii) in acetonitrile have been assigned " to the relaxation of coupled equilibria involving planar and tetrahedral isomers of the complex together with ion pairs and free ions. The kinetics of the rapid square-planar-octahedral interconversion accompanying the addition of two unidentate ligands L to (14) in chlorobenzene are consistent with a two-step mechanism,... 

Y. Ihara, A. Wada, Y. Fukada and K. Sone, Preparation and thermal square planar-octahedral transformation of nickel complexes containing 1,2-butanediamine or 3,3-dimethyl-l,2- butanediamine in solid phase. Bull. Chem. Soc. Jap. 59 1037 (1986). 

The dynamics of the square-planar - octahedral isomerization of a nickel(II) complex with N-(2-piperidinoethyl)salicylaldiminato ligands has been studied by ultrasonic relaxation methods- O This rearrangement can be represented as follows. Scheme 11.8 ... 

Simple nickel salts form ammine and other coordination complexes (see Coordination compounds). The octahedral configuration, in which nickel has a coordination number (CN) of 6, is the most common stmctural form. The square-planar and tetrahedral configurations (11), iu which nickel has a coordination number of 4, are less common. Generally, the latter group tends to be reddish brown. The 5-coordinate square pyramid configuration is also quite common. These materials tend to be darker in color and mostiy green (12). 

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. 

In addition to the tetrahedral and octahedral complexes mentioned above, there are two other types commonly found—the square planar and the linear. In the square planar complexes, the central atom has four near neighbors at the corners of a square. The coordination number is 4, the same number as in the tetrahedral complexes. An example of a square planar complex is the complex nickel cyanide anion, Ni(CN)4-2. 

In a nickel-containing enzyme various groups of atoms in the enzyme form a complex with the metal, which was found to be in the +2 oxidation state and to have no unpaired electrons. What is the most probable geometry of the Ni2+ complex (a) octahedral (b) tetrahedral (c) square planar (see Exercise 16.96) Justify your answer by drawing the orbital energy-level diagram of the ion. 

With 2-formylpyridine S-methyldithiocarbazate, planar, diamagnetic nickel(II) complexes with stoichiometry [Ni(6-H)A], where A = Cl, Br, I, and NCS, have been isolated [126]. Also, paramagnetic, octahedral [Ni(6-H)2] has been prepared and spectrally characterized. These complexes have NNS coordination as does the related S-benzyldithiocarbazate, 24, in [Ni(31-H)A], A = Cl, Br [165]. 

Picolylphenylketone S-benzyldithiocarbazate, 48, yielded paramagnetic [ Ni(48-H)A 2] (A = Cl, Br) and diamagnetic [Ni(48-H)I] [207]. All three compounds are non-electrolytes and the iodo complex is planar while the other two complexes involve sulfur bridging atoms and five-coordinate nickel(II) centers. All three complexes can be converted to monomeric, octahedral complexes by addition of pyridine, 2-picoline or quinoline. 

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