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Co-ordinate (Dative) Bonding
Co-ordinate bonding, also known as dative bonding
is a type of covalent bonding (i.e., electron sharing). However, the shared electron pair(s) are completely provided by one of the participants in the union, and not by contributions from the two of them.
The contributors of these shared electrons are either neutral molecules which contain lone pair(s) of electrons on one of their atoms, or negatively charged groups (radicals) with free electrons to donate.
Examples of these are: H2O, NH3 and CN-.
Transition metal complexes are formed by these groups donating electrons into the vacant d orbitals of transition metals. In this case, these donor groups are called ligands or coordinating groups or just donors.
Examples of such complexes are: hexacyano ferrate(II) ion, [Fe(CN)6]2+; tetraammina copper(II) ion, [Cu(NH3)4]2+; hexamine cobalt(II) ion, [Co(NH3)6]2+; silver(I) diamine ion, [Ag(NH3)2]+ and hexaaquo copper(II) ion, [Cu(H2O)6]2+. These complexes combine with oppositely charged particles (i.e., negative particles) by electrostatic force of attraction to form electrovalent or ionic compounds.
[Cu(NH3)4]2+ + 2Cl- → Cu(NH3)4Cl2.
Other very common associations you will find co-ordinate bonding is in
1. ammonium ion, NH4+. In this ion, ammonia molecule , NH3 combines by donating its lone pair of electrons into the vacant s orbital of hydrogen ion, H+, forming NH4+. The ammonium ion could then combine with a negative ion, e.g. Cl- by electrovalency to form an ionic compound, NH4Cl.
2. Hydronium ion, H3O+. Here, water molecule, H2O donates a lone pair of electrons into the vacant s orbital of hydrogen ion, H+.
H2O(l) + H+(aq) → H3O+(aq)
Note: the complete compounds eventually formed
comprises of the three types of bonding, i.e., covalent, co-ordinate and electrovalent.
Example: in NH4Cl: formation of NH3 (covalency); formation of NH4+ (co-ordinate or dative bonding); and formation of NH4Cl (electrovalency).
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