Octahedral molecules, main group

The only divalent main group ions where exchange rates and activation parameters are experimentally available are beryllium(II), which is also the smallest metal ion (rM = 27pm) (28,29) and magnesium(II) (r-yj = 72 pm). An important consequence of this substantial difference in ionic radii is that Be2+ forms predominantly tetrahedral complexes (30) and Mg2+ is in general surrounded by six solvent molecules forming octahedral complexes (31). 

Cluster of the series M5E often adopt a square-based pyramidal geometry with the main group element in the center of the square base. Such compounds often have seven skeletal electron pairs and are considered to be m do-octahedral molecules. The main group element often acquires a position slightly below the square of metal atoms. The position of the carbon atom in carbides apparently depends upon the charge of the complex. Thus in the compound Fe5(CO)i5C the carbon atom lies 0.09 A below the square base meanwhile in the anion [Fe5(CO)i4C] such deviation grows up to 0.18 A. In the series of compounds Ru5(CO)i5 x (PPh3),C these distances are 0.11,0.19, and 0.23 A for X = 0, 1, and 2 respectively. 

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