Inorganic complexes coordination numbers

PGM form complexes with a variety of organic and inorganic ligands. Depending on their valency, PGM elements prefer either coordination number 4 (planar complexes) or 6 (octahedral complexes). Coordination number 5 (trigonal-bipyramidal, square-pyramidal) and complexes with a tetrahedral structure can also be observed, but to a much lesser extent. Of special importance are chloro complexes of PGM, which play an important role in PGM extraction and refining processes and in analytical chemistry. 

The structure theory of inorganic chemistry may be said to have been bom only fifty years ago, when Werner, Nobel Laureate in Chemistry in 1913, found that the chemical composition and properties of complex inorganic substances could be explained by assuming that metal atoms often coordinate about themselves a number of atoms different from their valence, usually four atoms at the comers either of a tetrahedron or of a square coplanar with the central atom, or six atoms at the comers of an octahedron. His ideas about the geometry of inorganic complexes were completely verified twenty years later, through the application of the technique of x-ray diffraction. 

In inorganic and coordination chemistry, the Cu(II) state is the most abundant one, and is regarded as more stable than the Cu(II) state under normal conditions [32]. Although numerous examples of Cu(I) coordination complexes are known, their chemistry is rather limited and they are readily oxidized to Cu(II) species [32]. Of the common oxidation states, compounds derived from copper(III) are rare, with only 30-40 reported examples [32]. Despite the small number of isolated Cu(III) compounds, however, organocopper] 111) species have been proposed as important intermediates in copper-mediated organic reactions (Chapts. 4 and 10). 

The lanthanides in several complexes exhibit mixed (promiscuous) coordination numbers and geometries, similar to the presence of mixed oxidation states in a inorganic compound. We shall only discuss a few cases here to make the readers aware of this interesting situation. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

COORDINATION COMPOUNDS. One of a number of types of complex compounds, usually derived by addition from simpler inorganic substances. Coordination compounds are essentially compounds to which atoms or groups have been added beyond the number possible on the basis of electrovalent linkages, or the usual covalent linkages, to which each of the two atoms linked donates one electron (o Form the duplet. The coordinate groups are linked to the atoms of the compound usually by coordinate valences, in which both the electrons in the bond are furnished by the linked atom of the coordinated group. The amines and complex cyanides are representative of coordination compounds. 

The Cahn-Ingold-Prelog rules work for inorganic compounds too but coordination complexes often have coordination numbers greater then four and may exhibit helical chirality, for example, denoted A and A (or Pand Min the Cahn-Ingold-Prelog system). The formal condition for chirality is that the molecule should not have an improper axis of rotation (i.e. a rotation + reflection axis, 5n =... 

---