Other Coordination Spheres

Although some compounds would appear to have a coordination number 3, only a few, on close inspection, actually do. For example, cesium trichlorocuprate(II), Cs[CuCl3], is actually made up of chloride-bridged chains of tetrahedral [CuCb] units. Genuine examples of coordination number 3 often involve large, bulky ligands. Two relatively simple examples are given in Table 3.1. 

This tetrahedral coordination compound contains no internal mirror plane and is therefore chiral. 

Coordination number 4, as we have seen, involves either tetrahedral or square planar geometries. The former is commonly found for various d or electronic configurations, whereas the latter, as previously noted, is most often found with d and, occasionally, d metals. Often, the energetic difference between these two configurations is very small. A few representative examples of both geometries are given in Table 3.1. The number of coordination compounds with a coordination number of 4 is second only to those in which the coordination number is 6. 

Coordination number 5 also has two predominate geometries, square pyramidal and trigonal bipyramidal, that differ only slightly in energy. In fact, such five-coordinate compounds are often examples offluxional compounds, those that exist in 

The Berry mechanism is used for the interconversion of trigonal bipyramidal and square pyramidal forms of Fe(C0)5, a represerrtative fluxional compound. A given ligand may be axial in one of the trigonal bipyramids but equatorial in another. 

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Among other coordination spheres which may be chiral, there is the trivial case of the cA-hw-bidentate octahedral systems such as ciy-[Co(en)2Cl2], 6is-bidentate tetrahedral [M(A-A)2] or Z A-tridentate octahedral [M(A-A-A)2] complexes where the dihedral angle between the two chelate planes is different ih>m 90°, as is often the case. An example, together with the tns-tridentate [M(A-A-A)3] tricapped trigonal prism geometry often found for lanthanide complexes, is shown in Figure... 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

As with other crystalline substances, on heating coordination compounds may melt, sublime, decompose, or undergo a solid phase transition. The greater complexity of the constituents present increases the number of types of bond redistribution processes which are, in principle, possible within and between the coordination spheres. The following solid-state transitions may be distinguished (i) changes in relative dispositions... 

This is an example of a substitution reaction, a reaction in which one 1 ewis base takes the place of another. Here the CN ions drive out H20 molecules from the coordination sphere of the [Fe(H20)6]2+ complex and take their place. Replacement is less complete when certain other ions, such as Cl, are added to an iron(II) solution ... 

Although they are built from the same numbers and kinds of atoms, structural isomers have different chemical formulas, because the formulas show how the atoms are grouped in or outside the coordination sphere. Stereoisomers, on the other hand, have the same formulas, because their atoms have the same partners in the coordination spheres only the spatial arrangement of the ligands differs. There are two types of stereoisomerism, geometrical and optical. 

It is helpful in the discussion to describe silicate structures using the Q nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO ] " are represented as Q and those fully connected to other Q units as Q. In general, minerals based on Q , Q and units are decomposed by acids. Such minerals are those containing isolated silicate ions, the orthosilicates, SiO (Q ) the pyrosilicates, Si O " (Q ) ring and chain silicates, (SiOg) (Q ). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2 3 when attack occurs at A1 sites, with scission of the network (Murata, 1943). 

The ligand field of a quasi-octahedral coordination sphere acting on the (i-orbitals lifts the degeneracy of the excited spin quartet (and others) as sketched in tier (3). The nature of the resulting states and their energy depends on the strength... 

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