A #"coordinate bond"# is described when the two electrons, that form or define the bond, ARE formally derived from the ONE atom. In coordination chemistry examples of coordinate bonds are legion, given that ligands, which possess one or more lone pairs of electrons, are conceived to #"coordinate to"#, to donate electrons, to bind to a Lewis-acidic centre, typically a metal, i.e. #M^(n+)#, to form a so-called #"coordination complex"#; for example, #[M(NH_3)_6]^(n+)#, etc.
The simplest coordinate bond is formed between ammonia and the proton, as seen in the following acid-base reaction.....
#NH_3(aq) + H_2O(l) rightleftharpoons NH_4^+ + HO^-#
In the ammonium cation, #NH_4^+#, ALL the #N-H# bonds are equivalent. However, for one of the #N-H# bonds, the contributing electrons are conceived to derive SOLELY from the nitrogen atom, and this is what we mean we say the nitrogen is #"quaternized"# in #"ammonium cation"#, and why we represent the nitrogen centre as a formal cation, #stackrel(+)NH_4#. Likewise, when ammonia binds to say a #Co^(3+)# centre, we say that each #ColarrN# bond is a coordinate covalent bond to give #[Co(NH_3)_6]^(3+)#.
Confused yet?
