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

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  • $\begingroup$ Guess they mean coordination compounds. $\endgroup$ – Ivan Neretin Mar 21 '17 at 12:12
  • $\begingroup$ Can you give me an example? $\endgroup$ – AdiC Mar 21 '17 at 12:25
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    $\begingroup$ Basic beryllium acetate will do. $\endgroup$ – Ivan Neretin Mar 21 '17 at 12:34
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    $\begingroup$ 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
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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 new 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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The maximum covalency of oxygen is just 2 . However , it also shows other valencies like 1 in peroxides and 1/2 in superoxides .I suggest that you should reconsider your text. The reason is that oxygen is a member of period 2. It does not have a vacant d orbital hence no shifting of electron or excitation of electron takes place. On the other hand , yes ,various elements shows covalency of 4 like lead etc . Some elements even shows covalency of 7 like chlorine . This tendency generally decreases down the group due to inert pair effect .

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    $\begingroup$ Please revise your typography. In English, commas, full stops, exclamation marks and question marks are typeset compress to the previous word, i.e. there is no space preceeding those punctuation marks. However, there is a single space following. (Older English typography suggests a double space after a full stop; for completeness’ sake.) $\endgroup$ – Jan Mar 21 '17 at 16:48
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    $\begingroup$ This is not even wrong, because this answer is about oxidation state. $\endgroup$ – Mithoron Mar 21 '17 at 18:01
  • $\begingroup$ And without proper (negative) signs to boot. $\endgroup$ – Oscar Lanzi Mar 26 '17 at 17:58

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