Coordinate covalent bond
A coordinate covalent bond (formerly also known as dative bond, now little used) is a description of covalent bonding between two atoms in which both electrons shared in the bond come from the same atom. The distinction from ordinary covalent bonding is artificial, but the terminology is popular in textbooks, especially those describing coordination compounds. Once the bonds have been formed using this, its strength and description is no different from that of other polar covalent bonds.
Coordinate covalent bonds occur when a Lewis base (an electron donor or giver) donates a pair of electrons to a Lewis acid (an electron acceptor) to give a so-called adduct. The process of forming a dative bond is called coordination. The electron donor acquires a positive formal charge, while the electron acceptor acquires a negative formal charge.
- 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.
- beryllium dichloride (BeCl2) is described as electron deficient in the sense that the triatomic species (which does exist in the gas phase) features Be centers with four valence electrons. When treated with excess chloride, the Be2+ ion binds four chloride ions to form tetrachloroberyllate anion, BeCl42-, wherein all ions achieve the octet configuration of electrons.
Coordinate bonding is popularly used to describe coordination complexes, especially involving metal ions. In such complexes, several Lewis bases "donate" their "free" pairs of electrons to an otherwise naked metal cation, which acts as a Lewis acid and "accepts" the electrons. Coordinate bonds form and the resulting compound is called a coordination complex, and the electron acceptors are called ligands. A more useful description of bonding in coordination compounds is provided by Ligand Field Theory, which embraces molecular orbitals as a description of bonding in such polyatomic compounds.
Many chemical compounds can serve as ligands, often these contain oxygen, sulfur, nitrogen, and halide ions. The most common ligand is water (H2O), which forms coordination complexes with metal ions (like the hexaaquacopper(II) ion, [Cu(H2O)6]2+). Ammonia (NH3) is also a common ligand, as well as anions, especially fluoride (F-), chloride (Cl-), and cyanide (CN-).