Coordination compound octahedral complexes

As in olefin coordination compounds, acetylene complexes may have trigonal, square-planar, octahedral, trigonal-bipyramidal, and tetrahedral structures. In four-coordinate square-planar complexes, the acetylene molecule is perpendicular to the plane which comprises the remaining ligands and the central atom, while in trigonal and... 

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). 

The chemistry of Cr(III) in aqueous solution is coordination chemistry (see Coordination compounds). It is dominated by the formation of kineticaHy inert, octahedral complexes. The bonding can be described by Ss]] hybridization, and HteraHy thousands of complexes have been prepared. The kinetic inertness results from the electronic configuration of the Cr ion (41). This type of orbital charge distribution makes ligand displacement and... 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

This is the most common coordination number for complexes of transition elements. It can be seen by inspection that, for compounds of the type (Ma4b2), the three symmetrical structures (Fig. 19.6) can give rise to 3, 3 and 2 isomers respectively. Exactly the same is true for compounds of the type [Mayby]. In order to determine the stereochemistry of 6-coordinate complexes very many examples of such compounds were prepared, particularly with M = Cr and Co , and in no case was more than 2 isomers found. This, of course, was only negative evidence for the octahedral structure, though the... 

Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

In polymerizing these compounds, a reaction between a-TiCls and triethylaluminum produces a five coordinate titanium (111) complex arranged octahedrally. The catalyst surface has four Cl anions, an ethyl group, and a vacant catalytic site ( ) with the Ti(lll) ion in the center of the octahedron. A polymerized ligand, such as ethylene, occupies the vacant site ... 

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. 

In class I compounds (or complexes) the two sites are very different from each other and the valences are strongly localized. The properties of the complex are the sum of the properties of the constituting ions. The optical MMCT transitions are at high energy. The compounds are insulators. Here are some examples [60, 97]. In GaCl2, or Ga(I)[Ga(III)Cl4] there are dodecahedrally coordinated Ga(I) ions with Ga-Cl distances of 3.2-3.3 A and tetrahedrally coordinated Ga(III) ions with Ga-Cl distance 2.2 A. In [Co(III)(NH3)6]2- Co(II)Cl4)3 there are low-spin, octahedrally coordinated Co(III) ions and high-spin, tetrahedrally coordinated Co(II) ions. For our purpose this class is not the most interesting one. 

Gs, trans, or metal effects occur when, e.g., an octahedral coordination compound ML6 [7] is transformed into a new complex, either MLSX [2], where a ligand has been altered, or M L6 [5], where the metal M has been replaced by an isovalent metal M. The comparison of either [2] with [7], or [2] with [7], discloses either cis and trans effects or metal effects inasmuch as the properties of the ligands L are altered by this variation in the first case, the substituent effect of X is subdivided into the cis and trans effects. 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Iron in both the +2 and +3 valence states forms several stable hexacoordi-nated octahedral complexes with cyanide (CN ) ion, known as ferrocyanide or hexakiscyanoferrate(4—), [Fe(CN)6] and ferricyanide or hexakiscyanofer-rate(3-), [Fe(CN)6]3-, respectively. The simple iron(II) cyanide, Fe(CN)2 is unstable and all iron cyanide compounds known are coordination complexes. 

Although the model has been extended to other types of coordination compounds [83, 84], namely d metal octahedral complexes such as [MCl6- (NCMe) ] +" (M = Nb, Ta n = 0-2) [84], its application still remains little explored [82-84]. 

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