Complexes with coordination number six

Another possible six-coordinate arrangement is that of six ligands at the corners of a regular hexagon with the metal atom at the centre. It has been found, but only when the geometry is imposed by the structure of the ligand as for the at the centre of the 2-dibenzo-18-crown-6 ligand in Fig. 3.6. 

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Cordination numbers two, four and six are found in transition metal complexes, with coordination number six commonly found. 

Complexes with coordination number six tend to be octahedral, for example 2+... 

Consider a nucleophilic substitution reaction in a complex with coordination number six (CN = 6) octahedral (Oh) substrate MX5Y occurring via SN mechanism. 

Various adducts of T1X3 have been noted earlier in this chapter, and a number of anionic complexes with coordination numbers of four, five or six have also been prepared. Salts of TlCl " and T1C13X (X = Br, I) are readily formed from T1X3 and (say) R NCl in non-aqueous... 

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. 

Covalent mono-, tri-, tetra-, and pentavalent hahdes tend to form anionic complexes with coordination numbers of two (equation 19), four (equations 20 and 21), and six (equations 22 and 23). Such reactions take place mostly through the addition of halide anions. 

All known Au compounds are diamagnetic ([Xe]4f " 5d low-spin configuration), and the large majority have square-planar geometry. However, Au complexes with coordination number five or six are also known. 

Because little is known about d" -complexes of coordination number six, the dihydri-dodichloroosmium complex OsH2Cl2(P Pr3)2 is focused on the viewpoint of coordination chemistry. The shape of the complex is better to understand as a trigonal prism rather than by octahedral geometry on the basis of both experimental and computational results [250]. Interestingly, this complex reacts with terminal alkynes to give car-byne complexes (eq (75)) [251]. 

Werner concentrated on the complexes of cobalt, chromium and platinum, with coordination numbers four and six. More recent studies have involved most metals of the periodic table, and complexes with coordination numbers from two to twelve or even higher have been investigated. An additional spur to the recent rapid development of coordination chemistry has been the realisation of the important role played by complexes in catalysis and in biological processes. All the metals in the periodic table from vanadium to zinc, as well as several others such as molybdenum and magnesium, are involved in life processes. A thriving new branch of the subject, bioinorganic chemistry, has developed to study these complexes. 

The most common coordination numbers for transition metal complexes of relevance in catalysis are four, five, and six. However, a few metal complexes with coordination numbers two, three, and seven are also of relevance. Structures 2.3-2.8 show the common coordination geometries for coordination numbers four, five, and six. 

To this point the complexes considered have shared the coordination number six and approximate octahedral geometry. It has been argued that they also share the dissociative reaction mode. There are examples of reactions both with and without intermediates of reduced (that is, 5) coordination, but the insensitivity to entering ligands is a consistent feature. It will be interesting, shortly, to see if the dissociative pattern persists in more or less organometallic octahedral systems but first we shall give some attention to the non-labile square planar systems. 

The other type consists of complexes A [Fe(R2C fc)3 ] (A = R4N, PPh ) in which the metal has the coordination number six (61). These compounds are very air-sensitive and their stability is dependent on the positive ion. Whereas with [Et4N ] the complex is easily transformed to Fe(R2dtc)2, with [PPh ] the compound is more stable (61). 

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