Co-ordinate covalent bond (or) Dative bond:The formation of Co-ordinate covalent bond was explained by "Sidwick"Definition: Dative bond is the bond formed by sharing of electron pair but the shared pair is contributed by one atom.Co- ordinate covalent bond is semi polar bond.Characteristics of dative bond:The atom which contributes the electron pair for sharing is called electron pair donor.The donor atom must have at least one pair of electrons in valence shell.An atom which accepts electron pair is called electron pair acceptor. It should have vacant orbital in valence shell.Co-ordinate covalent bond shown by an arrow "→" pointing from donor to acceptor.Let us study the concept with an example.Consider the formation of ammonium ion - NH4 Plus:In Ammonia molecule nitrogen has atomic number 7.The valence shell electronic configuration of nitrogen is 1S2 2S2 2Px1 2Py1 2Pz1.Nitrogen undergoes Sp3 hybridization with one lone pair of electron, one hybrid orbital, other three hybrid orbitals form three .bonds with three hydrogen atoms.In hydrogen ion - 1S vacant orbital is present.Therefore the nitrogen atom has one sp3 orbitals with lone pair which overlaps the vacant 1S orbitals of H+ ion to form co-ordinate covalent bond.Co-ordinate bond is rigid and directional like covalent bond.

Properties of compounds with Co-ordinate covalent bond:They are gases or liquids due to weak inter molecular forces.The melting & boiling points of co-ordinate compounds are higher than covalent compounds, but lower than ionic compounds.Co-ordinate compounds are soluble in non polar solvents like benzene and carbon tetrachloride. They are insoluble in polar solvent like water.They are stable as covalent compounds. They undergo molecular reactions which are slow.They exhibit isomerism due to directional nature of bond.Examples: Formation of ammonia boron trifluoride molecule:In ammonia molecule nitrogen atom donates a pair of electrons to the boron fluoride molecule and shares them. A dative bond is formed between them. H3N → BF3

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