Complexes with coordination number four

As mentioned before, Copper prefers macrocyclic chelators like cyclam-14 and its derivatives. Cyclam-14 forms thermodynamically stable complexes and its X-ray structure shows a square-plane configuration with four equal Cu-N bonds (Tasker and Sklar 1975). This complex with coordination number four lose less than 0.5% metal when incubated for 24 h in human serum. Open chain chelators do not reach this stability. Its bifunctional derivative CPTA (O Fig. 45.17) coupled to an antibody and labeled with Cu shows a half-life for copper exchange in phosphate buffer >1,000 days (Smith-Jones et al. 1991). 

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. 

FIGURE 20.12 The arrangement of ligand donor atoms (L) in ML complexes with coordination numbers 2,4, and 6. In the octahedral arrangement, four ligands are at the comers of a square, with one more above and one below the plane of the square. 

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. 

Complexes with coordination numbers of 4 are typically either tetrahedral or square planar. The tetrahedral geometry (Fig. 8.18a) predominates for four-coordinate complexes of the early transition metals (those toward the left side of the d block of elements in the periodic table). Geometric isomerism is not possible for tetrahedral complexes of the general form MA2B2, because all four tetrahedral sites are completely equivalent. 

Imidazole forms coordination complexes with transition metal ions with coordination numbers four or six (see CHEC-I). The modes of binding have been outlined <87MI 302-02,93AHC(58)123>. This... 

Rotation of the allyl group, in which syn-anti interconversion does not occur, is now known in many instances for complexes with coordination numbers greater than four (see Table III). NMR spectra consistent with allyl rotation in most square palladium complexes have been shown to result from ligand exchange (47). 

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

The polyaminocarboxylate-derivative of cyclam, TETA, forms Cu-complexes with coordination number 6 with involvement of four amino groups and two pendant carboxylates (Moi et al. 1987). Surprisingly, Jahn-Teller distortion involves two amines and not the carboxylates as expected. Again, the Copper-complex coupled to an antibody loses less than 1% of its metal per day when incubated in human serum. 

Mononuclear thiolate complexes containing nickel sites in different oxidation state as [Ni(SC5H40)2] and [Ni(SC6H40)2] were described by KOckerling and Henkel (1993). With respect to the chemical as well as electrochemical properties of the nickel sites of various dehydrogenases, [Ni(SC6H40)2] is a relevant model complex of coordination number four. 

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. 

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

Coordination Number Four. These complex ions either have (a) a square planar geometry with the four ligands at each corner, such that the metal ion lies in-plane at the center, or (b) a tetrahedral complex where the centrally located metal ion has four ligands arranged as the hydrogen atoms in methane. Square planar complexes of... 

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). 

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