Is $\ce{BCl5}$ possible? I saw it in book. As far as we can see there is no way to get 5 chlorine atoms with boron.

  • $\begingroup$ It appears that $BCl_5$ is not a stable compound and you're right that it is difficult to get 5 chlorides around boron. What you should ask is how can I get a neutral molecule when Boron can only have a charge of +3? $\endgroup$
    – LDC3
    Jul 19 '14 at 13:18

Boron pentachloride is likely not stable except perhaps in extreme conditions, such as under very high pressures. Even then it may be possible that a description such as $\ce{[BCl4^{-}]Cl^+}$ containing a tetrahedral boron anion could turn out to be more accurate than any hypercoordinate structure (a boron atom surrounded by more than four ligand atoms).

However, pentacoordinate and hexacoordinate boron are not unheard of. Indeed they are common in borane clusters, though this type of structure has unusual bonding and perhaps is best left in its own category. Even without considering clusters, it is still possible to create hypercoordinate boron atoms by exploring steric demands of certain ligands. Examples can be found in this article, among several others. A simple diagram exemplifying how to force hypercoordination in boron is given here; notice how a four-coordinate structure requires rotation of the C-C bond between the left and middle aromatic rings, decreasing the extent of $\pi$-conjugation between rings and causing that particular molecular geometry to be less stable relative to the five-coordinate structure. Hexacoordinate carbon has also been prepared with even higher steric forcing.

Additionally, it should be mentioned that "hypercoordinate" is the preferred term instead of "hypervalent". Not only is hypercoordination more general (all hypervalent compounds are hypercoordinate, while not all hypercoordinate compounds need be hypervalent), but hypervalency usually implies some form of octet expansion, which is nowadays not generally believed to be a correct description of bonding.

  • 3
    $\begingroup$ Kudos for mentioning hypercoordinate! I still wonder if it exists in gas phase. Most certainly not in the hypercoordinate form, but maybe as a Lewis acid base pair $\ce{Cl-Cl->BCl3}$. $\endgroup$ Jul 19 '14 at 13:45

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