Aluminium coordination number dependence

Table 5.2 shows the predicted and observed coordination numbers of common cations with O2-. The Si4+ cation occurs in fourfold or tetrahedral coordination. Aluminium is generally found in sixfold or octahedral coordination but also occurs in tetrahedral coordination in igneous minerals. Where die radius ratio is near the boundary between two types of coordination, the cation may occur in either coordination, depending on conditions during crystallization. High crystallization temperatures generally favor low coordination numbers. In high-temperature minerals A1 tends to assume fourfold coordination and to substitute for Si. At lower temperatures A1 tends to occur in sixfold coordination and A1 substitution for Si is tare. 

The structural chemistry of aluminium alkoxides has posed many problems, owing to the tendency of aluminium to attain coordination numbers of 4, 5, or 6 depending on... 

Akitt and Duncan have provided further Al NMR evidence for the structure of the tetramer in solution. The spectrum consisted of a very broad peak due to tetrahedral aluminium and a sharper peak due to the central octahedral aluminium in the required 3 1 ratio of intensities. Owing to the quadrupole moment of the Al nucleus, the line width of the NMR signal depends on the symmetry of the electric field and gives independent evidence of the coordination of the aluminium. Furthermore, the structures of a number of other aluminium alkoxides were elucidated in 1984 by Al NMR spectroscopy, which supported the earlier proposed structures of a dimer (2-VII) and linear trimer (2-XI). It may be mentioned that a tetrameric structure for aluminium isopropoxide [Al(OPr )3]4 ° and a dimeric structure for aluminium f-butoxide [Al(OBu )3]2 have been confirmed by X-ray crystallography. 

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