My text says it to be 4. I believe it's 2. Is there any compound where covalency of 4 is observed?

  • $\begingroup$ Guess they mean coordination compounds. $\endgroup$ Mar 21 '17 at 12:12
  • $\begingroup$ Can you give me an example? $\endgroup$
    – AdiC
    Mar 21 '17 at 12:25
  • 3
    $\begingroup$ Basic beryllium acetate will do. $\endgroup$ Mar 21 '17 at 12:34
  • 2
    $\begingroup$ Yeah basic beryllium acetate has four coordinate O and can be thought of as significantly covalent due do the polarisability of Be. There are also numerous three coordinate species containing oxonium ions eg Me3O+ $\endgroup$
    – RobChem
    Mar 21 '17 at 12:47
  • $\begingroup$ $\ce{H4O^{2+}}$ is also an option. $\endgroup$ Apr 19 at 12:07

We know the maximum covalent is at least 3 with such species as $\ce{H3O+}$. Also the pyrylium ion (https://en.m.wikipedia.org/wiki/Pyrylium_salt) has an oxygen covalency of 3 by forming a pi bond for the third bond.

Can we get 4-coordination? As mentioned by @Rob, yes. @Rob's comment states that basic beryllium acetate appears to have 4-coordinate, covalently bound oxygen.

And it check out. Basic beryllium acetate, $\ce {Be4O(C2H3O2)6}$, has the four-coordinate oxygen in the center, then four beryllium atoms coordinated to this oxygen at the vertices of a tetrahedron, then an acetate group bridging each edge of the tetrahedron. In addition to beryllium, zinc forms a similarly structured basic acetate, which can be obtained by heating normal zinc acetate (https://en.wikipedia.org/wiki/Zinc_acetate).

Might the central oxygen be mostly ionically bonded? Magnesium and heavier alkaline earth metals, which form more strongly ionic bonds with oxygen, are not known to form such a basic acetate structure. This corroborates the hypothesis that covalent bonding of the central, four-coordinate oxygen is involved.


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