Magnesium coordination number

The effect of small valence and large coordination number is further shown by the observation that silicon tetrahedra, which share comers only with aluminum octahedra, share edges with magnesium octahedra (in olivine, chondrodite, humite, clinohumite) and with zirconium polyhedra with coordination number eight (in zircon). 

Fig. 11.11, p. 112). Incommensurate structures related to bismuth-III are also observed for strontium and barium. Magnesium, calcium and strontium are remarkable in that they transform from the normal closest-packing of spheres to a body-centered packing upon exertion of pressure. Even more remarkable is the following decrease of the coordination number to 6 for calcium and strontium (Ca-III, a-Po type Sr-III, /3-tin type). 

A DFT study was performed in order to determine the structures of phenylcalcium hydride and its magnesium analog in donor solvents.391 A dimeric phenylcalcium hydride was found to be the most stable species in solution, but monomers or tetramers cannot be excluded at very low or very high concentrations. Hydride bridging is favored over phenyl bridging, and a coordination number of six is predicted to be dominant in solution. 

Thus, if a magnesium ion binds six oxygen atoms, the strength of each metal ligand interaction is 2/6 = 0.33. If the cation is sodium, with the same coordination number, the bond strength is lower, 0.17. The above equation can be rewritten as... 

Fig. 11. Stereo of the Mg (H20)e ion in the crystal structure of magnesium citrate decahydrate (coordination number 6) (Johnson, 1965). Oxygen atoms are stippled hydrogen atoms are small and white. Note that none of the hydrogen atoms points to the oxygen atom of another water molecule coordinated to the same Mg ion.

Finally, for MPV reduction we find nearly equal barriers for aluminium and gallium, and a much lower one for magnesium. Again, one should be careful not to attach too much value to these numbers, since for magnesium-catalyzed MPV, in particular, the metal will probably prefer a higher coordination number than 4. 

As has been outlined before, the preferred coordination geometry at magnesium in organomagnesium compounds is tetrahedral, although also organomagnesium compounds are known with either lower or higher coordination numbers. 

FIGURE 2. Left deprotonation of pVIg(OH2)6] lowers the coordination number of magnesium. Reprinted with permission from Reference 14. Copyright 2005 American Chemical Society. Right pentacoordinate geometry of Mg + in the solid-state structure of the Mg + —F —Pi complex of yeast enolase... 

The alkaline earths, particularly the lighter ones, have more definite water coordination, and a probable coordination number of 4. The heats of sublimation (Table I) of the magnesium salts are about 50 kcal/mole or higher, those for the calcium halides are over 60 kcal/mole, and for the heavier alkaline earths they run over 70 kcal/mole. For the beryllium halides other than the fluoride, however, the values are 30 to 33 kcal/mole. In... 

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