Nickel coordination number

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). 

Mononuclear Carbonyls. The lowest coordination number adopted by an isolable metal carbonyl is four. The only representative of this class is nickel carbonyl [13463-39-3] the first metal carbonyl isolated (15). The molecule possesses tetrahedral geometry as shown in stmcture (1). A few transient four-coordinate carbonyls, such as Fe(CO)4, have also been detected (16). 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

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. 

Mapsi et al. [16] reported the use of a potentiometric method for the determination of the stability constants of miconazole complexes with iron(II), iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The interaction of miconazole with the ions was determined potentiometrically in methanol-water (90 10) at an ionic force of 0.16 and at 20 °C. The coordination number of iron, cobalt, and nickel was 6 copper and zinc show a coordination number of 4. The values of the respected log jSn of these complexes were calculated by an improved Scatchard (1949) method and they are in agreement with the Irving-Williams (1953) series of Fe2+ < Co2+ < Ni2 < Cu2+ < Zn2+. 

These structures can, for steric reasons, be formed only when the coordination number of the nickel atom is 7 or less. We call this species the subcarbonyl species as it is an intermediate in the formation of Ni(CO)4-Further evidence on this point will be given below. 

Adsorption of carbon monoxide takes place all over the surface and there is distinct evidence that, at least on nickel, the CO stretching frequency depends upon the coordination number of the nickel atom to which it is attached. Hence, the adsorption of carbon monoxide yields information about the relative numbers of surface atoms with different coordination numbers. This information, howrever, is at best merely of a semiquantitative nature. Steric effects also play a role, as is evidenced by the fact that the subcarbonyl species can be formed only on nickel atoms with a lowr coordination number. 

The student was not told the coordination number of the compound, so n was used to represent the number of ammonia ligands surrounding the nickel(II) ion. After the reaction has reached completion, additional HC1 was then titrated back with a standardized NaOH solution ... 

Oxide ratio, 18 815 Oxides, 16 598 acidic, 22 190-191 bond strengths and coordination numbers of, 22 570t diorganotin, 24 819 glass electrodes and, 14 28 gold, 22 707 iron, 14 541-542 lead, 14 786-788 manganese, 15 581-592 nickel, 27 106-108 niobium, 27 151 plutonium, 29 688-689 in perovskite-type electronic ceramics, 14 102... 

The nitrogen in the ammonia and the oxygen in the water are the donor atoms. They are the atoms that actually donate the electrons to the Lewis acid. The coordination number is the number of donor atoms that surround the central atom. As seen above, the coordination number for Cr3+ is 6. Coordination numbers are usually 2, 4 or 6, but other values can be possible. Silver (Ag ) commonly forms complexes with a coordination number of 2 zinc (Zn2+), copper (Cu2+), nickel (Ni2+), and platinum (Pt2+) commonly form complexes with a coordination number of 4 most other central ions have a coordination number of 6. 

In order to give the usual overview of nickel complexes at increasing coordination numbers we begin with the usual square planar complexes of the Schiff bases salen and saloph.149,150 As an example, Figure 98 shows the molecular structure of [Nin(salen)]. 

Only in homoleptic M(L)2 (L = 1,3-dimesitylimidazolin-2-ylidene) of zero-valent nickel and platinum significantly shorter metal-carbon bonds for NHCs and, thus, metal-to-ligand back donation can be observed. The Ni-C bond length is about 0.15 A shorter than in [Ni(CO)2(L)2] (L = 1,3-dimesitylimidazolin-2-ylidene) which cannot be explained exclusively by the change of the coordination number. 

Nickel forms a large number of complexes with various anions (monoden-tate, bidentate, and polydentate) and many neutral ligands. The most common coordination numbers of the metal in these complexes are six and four while the metal is usually in +2 oxidation state, Ni2+. Also, some complexes of three and five coordinations exist. Several zero valent nickel complexes, such as nickel tetracarbonyl, and a number of substituted carbonyl complexes are well known. 

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