Co-ordinate-covalent bonding or Dative bonding
The electron contributions of combining atoms in a covalent bond are generally equal. In each shared pair of electrons one electron is contributed from each atom of the bond. However in some bond formation, the whole of the shared pair of electrons comes from only one of the combining atoms of the bond, which is to referred as the donor atom. The other atom which does not contribute the electron to the shared pair but tries to pull the pair of electron towards itself is called as the acceptor atom. The bond thus formed is between the donor and acceptor atoms is called as the co-ordinate or co-ordinate - covalent or dative bond.
A coordinate bond is showed as an arrow which points from the donor to the acceptor atom. In some cases, the donated pair of electron comes from a molecule as a whole which is already formed to an already formed acceptor molecule as a whole.
For Example, coordination bond between H3N: and BF3 molecules. The molecule, ammonia (donor) which gives a pair of electron (lone pair) to BF3 molecule which is electron deficient (acceptor) which has an empty orbital to accommodate the pair of electrons. Thus a dative bond is formed and the molecule as a whole is represented as H3N -- > BF3 (Fig. a).
When Proton is added to ammonia, a pair of electron is donated by nitrogen to proton and then proton shares the electron pair to form coordinate covalent bond.
Similarly in (NH4Cl) ammonium chloride, covalent - coordinate bond exists in NH4+ ion only and Cl- ion exists as it is.
Few examples of covalent - coordinate bond :
In nitro methane (CH3 - NO2), one of the N-O-bond exists in a covalent coordinate type.
Aluminum chloride Al2Cl6 (dimeric form)
Lone pairs of electron from chlorine are donated to electron deficient aluminium atoms in such a way that dimers of AlCl3 are formed easily (Fig. d). The two chlorine atoms act as bridge to link the two Aluminium atoms.
In some complex ion formations, if the central transition metal-ion has empty `d' orbitals then lone pair of electrons from neutral molecules or anions are donated resulting in the formation of coordination bonds. Example : In Nickel tetracarbonyl, the four bonds between central Ni atom and the carbonyl ligands are mainly covalent -coordinate type. This complex exists in square planar geometry.