Six-coordinate molecules

Reaction of acidic dichromate solutions with H2O2 produces a blue compound, which can be extracted into ether. Upon coordination of various donors complexes of the composition [Cr0(02)2(L)] (L = H2O, py (15), Et20, etc.) can be isolated. These six-coordinate molecules adopt roughly pentagonal pyramidal structures and contain two side-on bound peroxide hgands. Deprotonation of [CrO(02)2(0112)] yields the ion [Cr0(02 )2 (OH)] , which may be isolated in the form of violet explosive salts. Related to the latter are the blue peroxohalochromates, Ph3AsMe[Cr0(02)2X] (X = Cl, Br). 

Sulfur, selenium, and tellurium are all capable of forming six-coordinate compounds such as SEg. Give two reasons why oxygen cannot be the central atom in such six-coordinate molecules. 

A rapid equilibrium between five- and six-coordinate molecules, as shown by Janzen and colleagues (353) (eq. [115]), can be responsible for the isomeriza-... 

Green-black five-coordinate [FeNO(S2CNR2)2] is reportedly converted into the dark brown six-coordinate molecule [Fe(NO)i(S2CNRi)2] by action of NO in chloroform. Subsequent investigation, however, has failed to confirm this although some evidence for the existence of the molecule as an intermediate was gained from " N/ N exchange studies. 

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