What are coordinate covalent bonds?

Answer 1

this is a strange bond that forms (for example) with CO, carbon monoxide when the octet rule cannot be satsified by ordinary means. covalent bonds require all atoms get a full orbital of electrons. they share the valence electrons to do so. Carbon has 4 valence electrons, oxygen has six. to combine them together, with carbon dioxide, it's easy to see how the carbon shares two valence electrons with each oxygen, and the oxygen shares two valence electrons back to the central carbon, both oxygen and carbon share 8 electrons with carbon being double bonded to both oxygens. with carbon monoxide, carbon shares 2 electrons in a double bond to oxygen because oxygen only needs to get 2 electrons. this leaves carbon "short" electrons. carbon has 4 valence electrons, borrows 2 back in the double bond from oxygen, but this leaves only six for carbon which is unstable. so... oxygen shares a pair of its unbonded electrons with the carbon as well, the coordinate covalent bond. usually electrons are shared by both atoms to form a bond. the C=O double bond does not make carbon stable, so the carbon octet is made possible by oxygen sharing 2 of its electrons, but both of these electrons are "oxygen" only electrons.