5. Resonance 6. Ionic & Dative Bonds 7. 3D Coupling Diagrams
6. Ionic and Dative Bonding
Ionic Bonding

Ionic bonding occurs when the electronegativity difference between two atoms favors the full or nearly full transfer of an electron to the more electronegative atom. It is similar to the polarization that occurs in a polar covalent bond, but there is a qualitative difference between an ionic bond and a polar covalent bond.

The difference is best seen in the total electron density of the molecules (the square of a real wavefunction or Ψ*Ψ for a complex wavefunction). Here are the total densities for H2, HF, and LiF:

In covalently bonded H2 and HF, there is significant electron density beween the two nuclei. In LiF, the electron density between the two nuclei is much reduced. LiF looks like spherical Li+ and F entities. It is worth noting that the fluorine in HF does not look that different from the one in LiF.

The coupling digram for LiF is just the two ions, with no coupling between the glyphs because there is no shared pair.

Dative Bonding

In contrast to ionic bonds, dative bonds do involve a shared pair of electrons. But in contrast to polar bonds, one of the entities contributes both electrons. The other entity has an orbital that is not being used, which allows the sharing to occur. In §2, we saw that a helium atom will share its 1s2 pair with a proton to form HeH+. The sharing is not equal, since the nuclear charge of He is twice that of H. Dative bonds can be represented with a coupling diagram that uses an arrow to indicate which pair interacts with which unoccupied orbital. Here is the coupling diagram for HeH+:

Formation of charged species via protonation is common in chemistry. An important example is hydronium (H3O+), where the proton interacts mostly with the 2s2 pair on water, as shown here:

Dative bonds can occur with entities other than protons. One of the most important examples is the formation of many oxyacids. Here are coupling diagrams for nitrous acid (HNO2 or HONO) and nitric acid (HNO3 or HONO2).

Nitrous acid involves only covalent bonds, including a π bond to the terminal oxygen atom. Adding the third electron involves a dative bond between the 2s2 pair on nitrogen and an empty 2p orbital on oxygen. This is oxygen in the 1D excited state rather than the ground state.

The final example of dative bonding is the binding that occurs in coordinated complex compounds, such as the aluminum hexaaquo trication, [Al(H2O)6]+3, shown here:

With its valence electrons stripped, aluminum has empty 3s and 3p orbitals that allow the 2s2 pairs on the six water molecules to be shared with the metal ion.

§7 provides a number of examples of 3D coupling diagrams.

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7. 3D Coupling Diagrams
Copyright 2023 DE Woon, University of Illinois. All rights reserved.