Molecules having ionic,covalent&coordinate covalent bonds?

Question

Molecules having ionic,covalent&coordinate covalent bonds?

Answer 1

Modern bonding theories like molecular orbital theory no longer recognize "coordinate covalent bonds".  The reason is that in MO theory and valence bond theory, all valence electrons "belong" to the entire molecule, rather than a pair of electrons "belonging" to a single atom.

We can say that all bonds are covalent.  It's just that some covalent bonds are really, really polar.  We say that the really polar bonds are "ionic".  But keep in mind that there are no 100 percent ionic bonds.  Even the most ionic bond possible, Cs-F, is 8 percent covalent.

As a matter of convenience we say that bonds are predominately ionic when the electronegativity difference is 2.0 or greater.    Even with a DEN of 2.0, the bond is 63 percent ionic.

percent ionic character = 100(1 - e^(-DEN/4))

Often people make the erroneous assumption that bonds which are ionic are stronger than covalent bonds.  Wrong.  Some of the strongest bonds known are covalent bonds.

A lot of these misconceptions arise from the melting points of compounds which have high ionic character.  What makes these compounds have high melting points is the existence of a regular network of atoms.  It is the existence of the network, regardless of whether the bonds are predominately ionic or covalent that account for the high melting point.  

The substance with the highest melting point is diamond and it contains 100 percent covalent bonds, but it is a network solid.  SiO2 is close to diamond in terms of MP and it contains covalent bonds as well.  SiO2 is also a network solid.  It's the existence of the network that makes for the high melting point, not the bond type.

Summary:

Bonds which are 100% covalent are found in the diatomic molecules: H2, O2, N2, F2, Cl2, Br2, and I2.  Here the electronegativity difference is zero.

There are no 100% ionic bonds.  The most "ionic" bond is found in cesium fluoride.

There are no coordinate covalent bonds because we don't consider electron pairs as "belonging" to individual atoms, but rather to the entire molecule.

Answer 2

Ionic= NaCl

Covalent= HCl

Coordinate= PCl5