Complex ions coordination number

Complex Ions Coordination Numbers, Geometries, and Ligands... 

The chemistry of Th(IV) has expanded greatly since the mid-1980s (14,28,29). Being a hard metal ion, Th(IV) has the greatest affinity for hard donors such as N, O, and light haUdes such as F and CF. Coordination complexes that are common for the t7-block elements have been studied for thorium. These complexes exhibit coordination numbers ranging from 4 to 11. 

The richness of coordination chemistry is enhanced by the variety of shapes that complexes can adopt. The most common complexes have coordination number 6. Almost all these species have their ligands at the vertices of a regular octahedron, with the metal ion at the center, and are called octahedral complexes (1). An example of an octahedral complex is the hexacyanoferrate(ll) ion, [Fe(CN)f, 4. ... 

Many other shapes are possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), Hg(CI l,)2 (4), which is a toxic compound formed by bacterial action on aqueous solutions of I Ig ions. Coordination numbers as high as 12 are found for members of the / block, but they are rare in the d block. One interesting type of d-mctal compound in which there are 10 links between the ligands and the central metal ion is ferrocene, dicyciopentadi-enyliron(O), [Fe(C5H5)2] (5). Ferrocene is an aptly named sandwich compound, with the two planar cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

Coordination compounds are also known as coordination complexes, complex compounds, or simply complexes. The essential feature of coordination compounds is that coordinate bonds form between electron pair donors, known as the ligands, and electron pair acceptors, the metal atoms or ions. The number of electron pairs donated to the metal is known as its coordination number. Although many complexes exist in which the coordination numbers are 3, 5, 7, or 8, the majority of complexes exhibit coordination numbers of 2, 4, or 6. 

The total number of bonds from the ligand to the central metal ion is known as the coordination number. In the Ni -EDTA complex, the coordination number is six. 

Cobalt in its trivalent state forms many stable complexes in solution. In these complexes, the coordination number of Co + is six. The Co2+ ion also forms complexes where the coordination number is four. Several complexes of both the trivalent and divalent ions with ammonia, amines, ethylene diamine, cyanide, halogens and sulfur ligands are known (see also Cobalt Complexes). 

They based this modification on the known adsorbance of OH on glass and on the common occurrence of transition metal mixed water-ammonia complexes with coordination number of 4. Parallel stractural studies of the deposited CdS showed textured growth, supporting a mechanism whereby alternate Cd and S species were involved, in an ion-by-ion process. Such a growth suggests adsorption of a molecular hydroxy-ammine species rather than a cluster. In fact, the mechanism of Ortega-Borges and Lincot also does not differentiate between a hydroxide cluster and molecule. 

Most of the nickel compounds in the solid state and almost all in aqueous solution contain the metal in the oxidation state +2, which, by consequence, can be considered the ordinary oxidation state for nickel in its compounds. The electronic structure and stereochemistry of nickel(II) were reviewed in 1968.6 The most stable electronic configuration of the free Ni ion is [Ar]3d8 which is also the ground state configuration in its complexes. The overwhelming majority of nickel(II) complexes have coordination numbers of four, five and six. Complexes with coordination numbers of three, seven and eight are still quite rare. 

The next most common complexes have coordination number 4. There are two common shapes for this coordination number. In a tetrahedral complex, the four ligands are found at the vertices of a tetrahedron, as in the tetrachlorocobaltate(II) ion, [CoCl4]2 (3). An alternative arrangement, most notably for atoms and ions with JH electron configurations such as Pt21 and Au3+, is for the ligands to lie at the corners of a square, giving a square planar complex (4). 

Unlike the exclusively 6-coordinate iron(iii) siderophore complexes, higher coordination numbers are possible with plutonium the complex of Pu" with desferrioxamine E (DEE a hexadentate iron-binding siderophore ligand) has shown it to contain 9-coordinate [Pu(dfe)(H20)3]+ ions with a tricapped trigonal prismatic geometry (Figure 11.13). 

It should be noted that complexing ions are characterized with definite steric structures. The most widespread drug complexes with coordination number 6 are built according to the type of the octahedron, with the coordinated groups oriented to the octahedron s peaks (Table 1). The drug complexes with coordination number 4 also can be built on the type of tetrahedron or on the type of plane (Table 1). 

One essential distinction between a proton complex and a metal complex is that the coordination number of protons is different from that of metal ions. The coordination number of the proton is 1 (in hydrogen bonding, can also exhibit a coordination number of 2). Most metal cations exhibit an even coordination number of 2, 4, 6, or occasionally 8. In complexes of coordination number 2, the ligands and the central ion are linearly arranged. If the coordi-... 

Sulfoxides. Table 44 presents some of the reported derivatives of actinyl ions coordinated by sulfoxide and related ligands. These ligands coordinate through the oxide oxygen atom in the equatorial plane of the actinyl ion. Coordination numbers about the metal can range from six to eight, as is typical for actinyl species. One of the most common geometries of complexes is... 

The directional preferences for coordination to the alkali metal and alkaline earth cations is obviously related to the number of substituents coordinated to the cation. As yet there is little predictability of the coordination number among these cations. For example, the first member of this series, the Li" cation, is the best characterized with well over 500 X-ray crystal structures containing this ion. Coordination numbers to Li ranging from two through seven and all values in between can be found. The Li cation is also found symmetrically w-complexed to the faces of aryl anions and to conjugated linear anions (see... 

Finally, linear complexes with coordination numbers of 2 are known, especially for ions with d ° configurations such as Cu, Ag", Au, and Flg. The central Ag atom in a complex such as [Ag(NFl3)2] in aqueous solution strongly attracts several water molecules as well, however, so its actual coordination number under these circumstances may be greater than 2. 

In a metal ion surrounded by other atoms, the d orbitals are at higher energy than they are in an isolated metal ion. If the surrounding electrons were uniformly distributed about the metal ion, the energies of all five d orbitals would increase by the same amount (a spherical crystal field). Because the ligands approach the metal ion from different directions, they affect different d orbitals in different ways. Here we illustrate the application of these ideas to complexes with coordination number 6 octahedral crystal field). 

Problem 2 Looking at the formula of the complex compound, Na3[AIF ], identify the ligand, central ion, coordination number and cation ... 

In spite of the preponderance of 6-coordinate complexes, other coordination numbers are known the ions [VCU] and [VBr4] are tetrahedral and are notable in that 4-coordination... 

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