Pentavalent coordination at boron

A number of transition states with pentavalent coordination at boron have been reported. For example, the reaction of the [BH3-CO] complex with or the hydrolysis of In two instances the hypervalent 

While unknowns still remain, Pizer has developed a plausible explanation for the observed increase in rate with tetrahedral borates over trigonal boron 

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Figure 11 A transition state with pentavalent coordination at boron would be required for the complexation of a ligand with a tetrahedral boronate anion. Yamashita et al. have recently reported the first single-crystal X-ray structure of a pentavalent boron compound, unequivocally confirming the existence of these hypervalent boron species.Atoms marked in red represent oxygen, pink boron and grey carbon. For clarity hydrogen atoms are not displayed.

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