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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.

Example: [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).