Coordination number six

Reactions with Alcohols. The tendency of titanium(IV) to reach coordination number six accounts for the rapid exchange of alkoxy groups with alcohols. Departure of an alkoxy group with the proton is the first step in the ultimate exchange of all four alkoxyls. The four-coordinated monomer is expected to react... 

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). 

For a long time the coordination number six at silicon was discussed as the ultimate limit, but meanwhile, the coordination numbers five, seven and ten have also been verified ... 

Cobalt forms many complexes in both the divalent and trivalent states. While the d Co2+ ion exhibits a coordination number of four or six in the trivalent state, the d Co3 ion mostly exhibits coordination number six. Also, trivalent cobalt forms more stable complexes than Co2+ ion, and there are many more of them. The most common donor atom in cobalt complexes is nitrogen. 

The ammines are therefore often classified into types with coordination numbers six, four, three, etc. 

A further simplification of Eq. (24) can be made by considering the symmetry of the crystal field. To see how this happens, we shall consider Vc for octahedral symmetry. This field would be given by six equal charges placed at R along the x, y, and z axes. Since the coordination number six is very common in transition-metal complexes, this type of crystal field occurs often. 

RING COMPOUNDS OF PHOSPHORUS WITH COORDINATION NUMBER SIX 537... 

Table 2 Proof of Configuration for Coordination Number Six by Isomer Counting 11...

These are soluble, sarcoplasmic calcium-binding proteins found in vertebrates. Invertebrates contain different sarcoplasmic calcium-binding proteins.247 Parvalbumins are low molecular weight (10 000-12 000), acidic proteins which contain two binding sites for Ca2+. The structure of parvalbumin has been determined, and details of the calcium sites are given in Table 6. XANES studies indicate that the calcium sites are similar to those in calmodulin, but different from troponin C.244,248 The two parvalbumin sites have different coordination numbers, six and eight.249... 

The coordination number of the chelated metal atom determines the number of linkages to functional groups. It is typically greater than the valency of the metal ion for example, the divalent ions of copper and nickel have coordination numbers of four, and the trivalent ions of chromium, cobalt, iron a coordination number of six. In the case of iron the coordination number six applies for the di- and trivalent forms. 

The most important chelate types are illustrated in Figure 2.7. Compounds in column A display a metal ion with coordination number four. The coordination number six is represented by the chelates of column B and C. Rows a, b, and c cover bi-, tri- and tetraftmctional or -dentate ligands, respectively. The types Ab and the types of column B contain additional monofunctional ligands to complete the coordination numbers of four and six, respectively. 

If we consider the fluorides, for example, which form pure coordination lattices (p. 33), then those from the alkaline earth metals with the exception of magnesium and beryllium crystallize in the fluorite structure, in which the cation is surrounded by eight fluorine ions for CaF2 and CdF2, which have the same structure, r+/r is 0.71 and 0.69 respectively just at the limit V 3— 1 — 0.73. The fluorides of other divalent ions, such as Mn, Fe, Co and Ni and also Mg, crystallize in a structure with coordination number six (rutile type). It is only for BeF2 that the ratio r+/r = 0.23 lies below the limit of this coordination number and it has a structure similar to that of cristobalite (Si02) with four neighbours (see also p. 66). 

---