dative covalent bond

In valence bond theory, this is a type of covalent bond in which both electrons come from the same atom, as opposed to a regular covalent bond in which one electron comes from each. Dative bonds can be formed by any species with a lone pair of electrons, and accepted by any species with a space for such a pair in its outer shell. Species which form them are acting as nucleophiles; those which receive them are electrophiles.

One example of a species with a dative bond is nitric acid, HNO₃. The nitrogen atom donates a pair of electrons to one of the oxygen atoms, a bond which can be represented with an arrow:

H
   \
    O - N = O
           ↓
           O

Other species with dative bonds include carbon monoxide (CO), the ammonium ion (NH₄⁺) and the oxonium ion (H₃O⁺). The coordination bonds by which ligands attach to transition metal ions are also dative.

It is important to note that the idea of a dative bond only makes sense in terms of valence bond theory; in molecular orbital theory, which is somewhat more complex, bonding is viewed in terms of bond order, and electrons are seen to delocalise over the whole molecular system, so the notion of an electron pair "coming from" a particular atom is meaningless. Thus MO theory sees carbon monoxide, for example, as having a bond order of three, and the corresponding Lewis structure for this system is simply C≡O.