Editing HSAB theory (section)

== Theory ==
{{multiple image
|title=Hard–soft trends for acids and bases
|align=right|total_width=420
|image1=hardsoftacids.png|alt1=Hard–Soft Trends for Acids|caption1=Acids
|image2=hardsoftbases.png|alt2=Hard–Soft Trends for Bases|caption2=Bases
}}
Essentially, the theory states that ''soft'' acids prefer to form bonds with ''soft'' bases, whereas ''hard'' acids prefer to form bonds with ''hard'' bases, all other factors being equal.<ref>{{Cite journal |last=Muller |first=P. |date=1994-01-01 |title=Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994) |journal=Pure and Applied Chemistry |language=en |volume=66 |issue=5 |pages=1077–1184 |doi=10.1351/pac199466051077 |issn=1365-3075|doi-access=free }}</ref> It can also be said that hard acids bind strongly to hard bases and soft acids bind strongly to soft bases. The HSAB classification in the original work was largely based on [[equilibrium constant]]s of Lewis acid/base reactions with a reference base for comparison.<ref>{{Cite journal |last=Pearson |first=Ralph G. |date=1963 |title=Hard and Soft Acids and Bases |url=https://pubs.acs.org/doi/abs/10.1021/ja00905a001 |journal=Journal of the American Chemical Society |language=en |volume=85 |issue=22 |pages=3533–3539 |doi=10.1021/ja00905a001 |issn=0002-7863|url-access=subscription }}</ref>

{| class="wikitable"
|+ {{nowrap|Comparing tendencies of hard acids and bases vs. soft acids and bases}}
! Property !! {{nowrap|Hard acids and bases}} !! {{nowrap|Soft acids and bases}}
|-
| atomic/[[ionic radius]]|| small || large
|-
| [[oxidation state]] || high || low or zero
|-
| [[polarizability]]  || low  || high
|-
| [[electronegativity]] (bases) || high || low
|-
| {{abbr|[[HOMO]]|highest-occupied molecular orbital}} energy of bases<ref name="IUPAC">[[IUPAC]], [http://www.iupac.org/reports/1999/7110minkin/h.html Glossary of terms used in theoretical organic chemistry], accessed 16 Dec 2006.</ref><ref name=Miessler>Miessler G.L. and Tarr D.A. "Inorganic Chemistry" 2nd ed. Prentice-Hall 1999, p.181-5</ref> || low || higher
|-
| {{abbr|[[LUMO]]|lowest-unoccupied mollecular orbital}} energy of acids<ref name="IUPAC" /><ref name="Miessler" /> || high || lower {{nowrap|(but more than soft-base HOMO)}}
|-
| affinity
| [[ionic bonding]]
| [[covalent bond]]ing
|}
<!-- When the above was converted from two bullet lists into a single table, the examples were removed because almost all were duplicated in the table below. The only exception, Cd<sup>2+</sup>, should probably be added to the table below -->
{|align="center"  class="wikitable"
|+ Examples of hard and soft acids and bases
!colspan=4 align="center"|Acids||colspan=4 align="center"|Bases
|-
!colspan=2 align="center"|hard||colspan=2 align="center"|soft||colspan=2 align="center"|hard||colspan=2 align="center"|soft
|-
|[[Hydronium]]||H<sub>3</sub>O<sup>+</sup>||[[Mercury (element)|Mercury]]||[[methylmercury|CH<sub>3</sub>Hg<sup>+</sup>]], Hg<sup>2+</sup>, [[Mercurous ion|Hg<sub>2</sub><sup>2+</sup>]]||[[Hydroxide]]||OH<sup>−</sup>||[[Hydride]]||H<sup>−</sup>
|-
|[[Alkali metals]]||Li<sup>+</sup>,&nbsp;Na<sup>+</sup>,&nbsp;K<sup>+</sup> ||[[Platinum]]||Pt<sup>2+</sup>||[[Alkoxide]]||RO<sup>−</sup>||[[Thiolate]]||RS<sup>−</sup>
|-
|[[Titanium]]||Ti<sup>4+</sup>||[[Palladium]]||Pd<sup>2+</sup>||[[Halogens]]||F<sup>−</sup>,&nbsp;Cl<sup>−</sup>||[[Halogens]]||I<sup>−</sup>
|-
|[[Chromium]]||Cr<sup>3+</sup>,&nbsp;Cr<sup>6+</sup>||[[Silver]]||Ag<sup>+</sup>||[[Ammonia]]||NH<sub>3</sub>||[[Phosphine]]||PR<sub>3</sub>
|-
|[[Boron trifluoride]]||BF<sub>3</sub> ||[[Borane]]||BH<sub>3</sub>||[[Carboxylate]]||CH<sub>3</sub>COO<sup>−</sup>||[[Thiocyanate]]||SCN<sup>−</sup>
|-
|[[Carbocation]]||R<sub>3</sub>C<sup>+</sup>||[[P-chloranil]]||C<sub>6</sub>Cl<sub>4</sub>O<sub>2</sub>||[[Carbonate]]||CO<sub>3</sub><sup>2−</sup>||[[Carbon monoxide]]||CO
|-
|[[Lanthanides]]||Ln<sup>3+</sup>||Bulk [[metals]]||M<sup>0</sup>||[[Hydrazine]]||N<sub>2</sub>H<sub>4</sub>||[[Benzene]]||C<sub>6</sub>H<sub>6</sub>
|-
|[[Thorium]], [[uranium]]||Th<sup>4+</sup>, U<sup>4+</sup>||[[Gold]]||Au<sup>+</sup>||||||||
|}

Borderline cases are also identified: borderline acids are [[trimethylborane]], [[sulfur dioxide]] and [[ferrous]] Fe<sup>2+</sup>, [[cobalt]] Co<sup>2+</sup> [[caesium]] Cs<sup>+</sup> and [[lead]] Pb<sup>2+</sup> cations. Borderline bases are: [[aniline]], [[pyridine]], [[nitrogen]] N<sub>2</sub> and the [[azide]], [[chloride]], [[bromide]], [[nitrate]] and [[sulfate]] anions.

Generally speaking, acids and bases interact and the most stable interactions are hard–hard ([[ionic bond|ionogenic]] character) and soft–soft ([[covalent bond|covalent]] character).

An attempt to quantify the 'softness' of a base consists in determining the [[equilibrium constant]] for the following equilibrium:

:BH + CH<sub>3</sub>Hg<sup>+</sup> {{eqm}} H<sup>+</sup> + CH<sub>3</sub>HgB

where CH<sub>3</sub>Hg<sup>+</sup> ([[methylmercury]] ion) is a very soft acid and H<sup>+</sup> (proton) is a hard acid, which compete for B (the base to be classified).

Some examples illustrating the effectiveness of the theory:
* Bulk metals are soft acids and are [[Catalyst poisoning|poisoned]] by soft bases such as phosphines and sulfides.
* Hard [[solvent]]s such as [[hydrogen fluoride]], [[water]] and the [[protic solvent]]s tend to [[Solvation|dissolve]] strong solute bases such as [[fluoride]] and [[oxide]] anions. On the other hand, dipolar [[aprotic solvent]]s such as [[dimethyl sulfoxide]] and [[acetone]] are soft solvents with a preference for solvating large anions and soft bases.
* In [[coordination chemistry]] soft–soft and hard–hard interactions exist between ligands and metal centers.