Question

Which of the following is the softest Lewis acid?

	a.	Ag+
	b.	Ac3+
	c.	Au+
	d.	As3+

So far from what i've found "soft" lewis acids have electronegativies around 2.2, they have a low or no charge, and they have a large cationic radius. Because Arsenic and Actinum have 3+ charges i've ruled them out so now I just have to determine which is the softer acid between Gold and Silver. The electronegativities of Gold and Silver are 2.54 and 1.93 respectively. The cationic radii for Gold and Silver in their current charge states are 151 and 129 respectively. Is there a formula for determining the "hardness" of a cation? My guess is that Gold is softer than Silver because it has a bigger radius but i'm not sure how electronegativity plays in to this determination. Help please!

Answer 1

In general, a "hard" acid will be small with a high positive charge, while a "hard" base will be small with highly electronegative electron donor atoms. Soft acids and bases will be the opposite. In general, hard acids like to interact with hard bases, and soft acids like to interact with soft bases.

Hardness can roughly be considered as (Ionization Energy- Electron Affinity)/2.

If we make a lot of estimations, we can consider extent of hardness to be (HOMO-LUMO)/2 or roughly half of the band gap.