Hard-Soft Acid Base Theory

Inorganic Chemistry

Ácidos y bases

Aceptores y donadores

El concepto de Lewis es más general y puede ser interpretado en términos de la

teoría de OM’s

Los orbitales frontera

HOMO/LUMOdefinen la química

de una especie

-+C O

C OM

C O M

CO es un donador sy un aceptor p

Una base es un donador de pares de electronesUn ácido es un aceptor de pares de electrones

Los aductos ácido-base de Lewis que involucran iones metálicosse llaman compuestos de coordinación (o complejos)

ácido baseaducto

La molécula de NH3

En la mayoría de las reacciones ácido-base las combinaciones HOMO-LUMO de los reactivos dan

lugar a nuevos orbitales HOMO-LUMO en el producto

Sin embargo, para que se formen nuevos orbitales de enlace y antienlace debe haber un traslape significativo (por simetría) y las

energías deben ser similares

¿Qué reacciones tienen lugar si las energías son muy diferentes?

Aún cuando las simetrías sean compatibles, hay muchas reacciones posibles dependiendo de las energías relativas.

LUMO

HOMO

Una base debe tener un par de electrones en su HOMO con la

simetría adecuada para interactuar con el LUMO del ácido

Si son muy diferentes como A-B ó A-E

no se forman aductos

Si las energías son similares como en A-C ó A-D se forman aductos

Hard and soft acids and bases

Hard acids or bases are small and non-polarizableSoft acids and bases are larger and more polarizableHalide ions increase in softness: fluoride < chloride<bromide<iodide

Hard-hard or soft-soft interactions are stronger (with less soluble salts) than hard-soft interactions (which tend to be more soluble).

Most metals are classified as Hard (Class a) acids or acceptors.Exceptions shown below: acceptors metals in red box are always soft (Class b). Other metals are soft in low oxidation states and are indicated by symbol.

Class (b) or soft always Solubilities: AgF > AgCl > AgBr >AgI

But…… LiBr > LiCl > LiI > LiF

Chatt’s explanationClass (b) soft metals have d electrons available for -bonding

Higher oxidation states of elements to the right of transition metals have more class b charactersince there are electrons outside the d shell.

Ex. (Tl(III) > Tl(I), has two 6s electrons outside the 5d making them less available for π-bonding)

For transition metals: high oxidation states and position to the left of periodic table are hardlow oxidation states and position to the right of periodic table are soft

Soft donor molecules or ions that are readily polarizable and have vacant d or π* orbitalsavailable for π-bonding react best with class (b) soft metals

Model: Base donates electron density to metal acceptor. Back donation, from acid to base, may occur from the d electrons of the acid metal into vacant orbitals on the base.

Tendency to complex with hard metal ions

N >> P > As > SbO >> S > Se > Te

F > Cl > Br > I

Tendency to complex with soft metal ions

N << P > As > SbO << S > Se ~ Te

F < Cl < Br < I

The hard-soft distinction is linked to polarizability, the degree to which a moleculeor ion may be easily distorted by interaction with other molecules or ions.

Hard acids or bases are small and non-polarizable

Soft acids and bases are larger and more polarizable

Hard acids are cations with high positive charge (3+ or greater),or cations with d electrons not available for π-bonding

Soft acids are cations with a moderate positive charge (2+ or lower),Or cations with d electrons readily availbale for π-bonding

The larger and more massive an ion, the softer (large number of internal electronsShield the outer ones making the atom or ion more polarizable)

For bases, a large number of electrons or a larger size are related to soft character

Hard acids tend to react better with hard bases and soft acids with soft bases, in order to produce hard-hard or soft-soft combinations

In general, hard-hard combinations are energeticallymore favorable than soft-soft

An acid or a base may be hard or softand at the same time it may be strong or weak

Both characteristics must always be taken into account

e.g. If two bases equally soft compete for the same acid, the one with greater basicity will be preferred

but if they are not equally soft, the preference may be inverted

Fajans’ rules

1. For a given cation, covalent character increases with increasing anion size.2. For a given anion, covalent character increases with decreasing cation size.3. The covalent character increases

with increasing charge on either ion.4. Covalent character is greater for cations with non-noble gas electronic configurations.

A greater covalent character resulting from a soft-soft interaction is relatedWith lower solubility, color and short interionic distances,

whereas hard-hard interactions result in colorless and highly soluble compounds

2

AI

2

AI

Dureza Absoluta(Pearson)

Electronegatividad absoluta de Mulliken (Pearson)

1

Blandura

EHOMO = -I

ELUMO = -A

Energy levelsfor halogensand relations between, and HOMO-LUMO energies

Hard-Soft Acid-Base Concepts Hard Lewis acids tend to combine

with hard Lewis bases, and soft Lewis acids tend to combine with soft Lewis bases

The important properties to consider in the classification of Lewis acids and bases are electronegativity, size and charge; the key features of many of our deliberations.

Soft Acids

heavy metal ions insoluble chlorides chemically inert fairly electronegative (1.9 - 2.54) large size (> 90 pm) low charge (+1 or +2)

Hard Acids

most metals electronegativity between 0.7 and

1.6 small (<90 pm) often highly charged (3+ or higher)

Soft Bases

C, P, As, S, Se, Te, Br, I donor atoms electronegativity between 2.1 and

2.96 radius > 170 pm

Hard Bases

O, F donor atoms very high electronegativity r ~ 120 pm typical examples are sulfate,

carbonate,silicate, acetate, alcohols, ketones

Principle of Maximum Hardness Reacting molecules will arrange their

electrons so as to be as hard as possible (the electrons are more tightly held to the atoms and are therefore less polarizable).

Generally, hard molecules are those with large HOMO-LUMO gaps, and soft molecules are those with small HOMO-LUMO gaps.

Relative Softness

The relative softness of soft acids can be estimated from the distance of the acid from Au, the softest atoms.

For 1st row metals, the 2+ state is borderline,the 3+ state is clearly hard and the 1+ state (only observed for traditional coordination complexes as Cu(I)) is soft.

Acids get softer as they get heavier.

Relative Softness

The softest bases are those with the lowest electronegativity and lie along the metal-nonmetal border.

Donor atom softness is also influenced by the substituent atoms in bases like phosphines and arsine.

Example of the use of the HSAB

Principle Predicting to which side and

equilibrium lies:

Cation

Type

Why

Nb5+ hard

High electronegativity and charge

Hg2+ soft Low electronegativity and charge; closer to Au

Example of the use of the HSAB Principle

Anions

Type

Why

S2- soft Lower electronegativity, large

O2- Hard High electronegativity, smallTherefore, products are favored since

they match hard with hard and soft with soft.

Strength and Softness

We need to characterize acid-base behavior with two parameters (at least)

We have addressed strength with Z2/r and other considerations

The general strength rule is that the strongest acid reacts with the strongest base, and the weakest acid reacts with the weakest base.