Coordinate Covalent Bond

In an ordinary covalent bond, BOTH combining atoms contribute one electron each, and the resulting pair is mutually shared. In certain other bond formations, however, only ONE of the two combining atoms supplies BOTH electrons of the shared pair -- the necessary electron pair is donated entirely by one atom, and then shared between both. This type of bond is called a coordinate covalent bond (or coordinate bond, or dative bond). The atom that donates the electron pair is the donor atom; the atom that accepts it is the acceptor atom. A coordinate bond is denoted by an ARROW, drawn starting at the donor atom and pointing toward the acceptor atom (rather than the plain line used for an ordinary covalent bond). (Later, in the study of coordination compounds, the donor atom will be called the ligand and the acceptor the central metal atom/ion.)

Example 1 — the ferrocyanide ion, [Fe(CN)₆]⁴⁻. Each of the six cyanide ions (CN⁻) donates a lone pair of electrons to form a coordinate bond with the iron centre (Fe²⁺); these electrons are then shared between Fe²⁺ and each CN⁻ (Fig 10.11). …