Metal high coordination number

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

Table 26.2 also reveals a diminished tendency on the part of these elements to form compounds of high coordination number when compared with the iron group and, apart from [Co(N03)4], a coordination number of 6 is rarely exceeded. There is also a marked reluctance to form oxoanions (p. 1118). This is presumably because their formation requires the donation of n electrons from the oxygen atoms to the metal and the metals become progressively... 

If the coordination number of a given complex of a first row transition metal ion exceeds six there seems to be a general stabilization of the high-spin configuration. To our knowledge, there are no examples of Fe(II) crossover complexes with such high coordination numbers. 

The first experimental information on the kinetic parameters for water exchange on a tetravalent metal ion was published in 2000 for U4+ and Th4+ (265,268,271). The coordination numbers for these two complexes were determined by EXAFS to be 10 1. Based on the high coordination number (there are no complexes known with unidentate ligands and coordination numbers larger than 10) a limiting associative mechanism (A) is unlikely and a d-activated mechanism is probable. Surprisingly,... 

Tab. 2.4-1. Summary of alkali metal (M) amalgams containing anionic" mercury clusters, extended mercury partial structures and/or high coordination number polyhedra.

Among these phases, characterized by high coordination numbers, typical representatives are Cr3Si-type phases, a phases and p phases, Laves phases. As for phases formed with semi-metals or non-metals we may point out the NiAs, NaCl, M0S2, MoSi2 types. 

High coordination numbers of the metal atom are favored for type I and type Ilia structures by the small coordination angles of the bidentate 82 ligand. High coordination numbers also protect the metal center from nucleophilic attack, an important factor for the stabilization of metal-metal bonds. 

With such high coordination numbers it is quite clear that there can be no possibility of covalency, because there are insufficient numbers of electrons. The difficulty is shown in the case of metallic lithium, with its body-centred cubic structure and coordination number of 14. Each lithium atom has one valency electron and for each atom to participate in 14 covalent bonds is quite impossible. 

An example of a high coordination number for cadmium is found in aquabis(4-aminobenzoato)cadmium(II),793b which involves seven-coordinate cadmium, with two chelating carboxylate groups, a water molecule and axial positions filled by amino groups from ligands bound to the neighbouring metal centres.794... 

COORDINATION NUMBER. The number of nearest neighbors of a given atom in a crystal structure. In covalent crystals, only those neighbors to which the atom is directly bonded are counted, and this number is usually 4 or less. In metals. Ihe coordination number may be as high as 12. as in the close-packed structures. 

---