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Coordinate covalent bonds are bonds on which both electrons from one atom.

But why does this happen? Some may think it is because one of the bonding atoms have strong electronegativity. But experimental evidence suggests otherwise.

For example, carbon monoxide. The EN of carbon is 2.6, and oxygen 3.4. This may lead you to think that carbon will donate a pair of electron, but on Princeton.edu:

Carbon monoxide (CO) can be viewed as containing one coordinate bond and two "normal" covalent bonds between the carbon atom and the oxygen atom. This highly unusual description illustrates the flexibility of this bonding description. Thus in CO, carbon is the electron acceptor and oxygen is the electron donor.

Nevertheless, in some cases like NO2, it does follows that the less electronegative atom domates electron. But why??

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    $\begingroup$ That simply follows from the fact that oxygen normally cannot have three bonds. With three bonds it has to donate electrons, else it would end up with more than 8 electrons in its outer shell. $\endgroup$ – Jori Dec 14 '14 at 20:51
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We can share the women, we can share the wine

We can share what we got of yours, cause we done shared all of mine

"Jack Straw" Grateful Dead

But seriously now, the most important thing in the Princeton.edu article (which explains at the bottom that its content comes from Wikipedia) is

"The distinction from ordinary covalent bonding is artificial"

Does $\ce{H2}$ come from a hydride and a proton or from two hydrogen atoms? The reason a molecule is stable is independent of a particular path by which it is made.

For $\ce{CO}$, it is only in your mind that oxygen brought more wine (electrons) to the party, but what really matters is that there is enough wine (electrons) at the party for everyone to be satisfied.

$\ce{CO}$ is isoelectronic with $\ce{N2}$ and $\ce{CN-}$; the reason for bonding is the same for each.

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Electronegativity affects coordinate covalent bonds; indeed atoms which accept electrons should be "electron hungry" enough to take them. However it stems mainly from that the acceptor doesn't have complete electron shell, thus acts as Lewis acid and donor acts as base.

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  • $\begingroup$ If electronegativity is a factor in the reason why coordinate covalent bonds occur together with the incomplete electron shell, then why doesn't a coordinate covalent bond occur in CO2 rather than the well-known double bond? $\endgroup$ – suse Feb 28 at 14:36
  • $\begingroup$ Electronegativity is important factor in hydration of first group cations. You seems to have some weird misconception; this one ? Or another? $\endgroup$ – Mithoron Feb 28 at 15:59

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