Homo lumo diagram5/21/2023 ![]() This explains, for example, that NH 3 is a Lewis base. Figure 4.1.6 Molecular orbital diagram of NH 3 So how can Lewis acids and bases and their reactions be viewed from an MO theory perspective? MO theory states that a molecule is a Lewis base when its HOMO is approximately non-bonding (Fig. Because dative bonds are covalent bonds, MO theory should be able to explain dative bonds. MO theory is a theory designed to explain covalent bonding. You may want to remember in this context that completely ionic bonds are not possible in general, and there must also be some degree of covalency due to the finite electronegativity difference between two elements. In this case the ionic bonds would be interpreted as highly polar dative bonds. The bonds between Ca 2 and OH - would be viewed as a dative bonds indicated as arrows pointing from O to Ca. Figure 4.1.5 Acid-base reaction between Ca 2 and OH -įor example Ca 2 can be viewed as a Lewis acid, and OH - as a Lewis base in a reaction that forms calcium hydroxide Ca(OH) 2 as the Lewis acid-base adduct. In this case the anion would be the donor and the cation the acceptor. The Lewis acid-base concept can even be used to explain bonding in ionic crystals. The formed Zn-O bonds are dative bonds indicated as arrows pointing from the O to the Zn. Figure 4.1.4 Acid-base reaction between Zn 2 and OH -įor example, Zn 2 acts as a Lewis acid when reacting with 4 OH - as a Lewis base to form tetrahydroxo zincate (2-) anions (Fig. The bond between the metal ion and ligand is a dative bond pointing from the ligand to the metal. The ligand is the Lewis base and the metal ion is the Lewis acid, the coordination compound is the Lewis acid-base adduct. The Lewis acid-base concept also explains bonding in coordination compounds, and the formation of coordination compounds from metal ions and ligands. The same reaction could also be viewed as a Broensted acid base reaction with the OH - ion as the proton acceptor, and the H ion as the proton donor. Of course, there is no actual difference between the two bonds, we just have a different perspective on them. In this case one of the O-H bonds in the water molecule would be viewed as a dative bond, while the other one would be viewed as a “regular” covalent bond. Figure 4.1.3 Acid-base reaction between H and OH -įor example, an H ion is a Lewis acid and an OH - anion is a Lewis base, and the two can react to form water as a Lewis-acid base adduct. Here are a few examples that can illustrate the generality of the Lewis acid-base concept. Not every Lewis acid is a Broensted acid, and not every Lewis base is a Broensted base. It includes the Broensted acid-base concept, meaning that any Broensted acid is also a Lewis acid, and any Broensted base is also a Lewis base. The reaction product of a Lewis acid-base reaction is called an adduct.Īs mentioned previously, the Lewis acid-base concept is quite general and can explain the bonding in quite different compounds. To indicate a dative bond one can draw an arrow pointing from the donor to the acceptor atom, instead of just a line. There is however no fundamental difference between a “conventional” covalent bond and a dative covalent bond, it is just a matter of perspective. In a "conventional" covalent bond both partners contribute one electron to the covalent bond. A covalent bond formed in a Lewis acid-base reaction is usually called a dative bond because both electrons in the covalent bond come from a single partner. The Lewis base donates an electron pair to form a covalent bond with the Lewis acid (Fig. What happens in a Lewis-acid base reaction? Figure 4.1.2 Acid-base reaction between NH 3 and BH 3 \)Īn example of a Lewis acid is BH 3, and an example for a Lewis base is NH 3. ![]()
0 Comments
Leave a Reply. |