Hard and Soft Acid and Bases

Lewis Acids & Bases A base is an electron-pair donor, and an acid is an electron-pair acceptor. The two combine to form an adduct. A + :B  A-B In complexes metals are electron acceptors hence they are acids and Ligands give lone pair of electrons and are bases. Metals were divided into two categories Class A/Type I Class B/Type II 1.Alkali and alkaline earths Transition metals lower OS 2. TM in higher OS Large size 3. Small size Less electropositive 4. Highly electropositive

Hard and Soft Acids Hard acids R.G. Pearson (1963) categorized acids and bases as either hard or soft (using Kf values). Hard acids Alkali and alkaline earths TM in higher OS Highly electropositive and very few electrons They are not very polarisable and prefer less polarisable ligands. Prefer complexes with ligands having donor atoms like N, O, F high charge density Their d orbitals are often unavailable to engage in π bonding. Hard Acids prefer to bond with Hard Bases

Hard and Soft Acids Soft acids Transition metals lower OS Large size Less electropositive Polarisable Their d orbitals are available for π bonding. often 2nd and 3rd row transition metals with a +1 or +2 charge filled or nearly filled d orbitals Prefer complexes with ligands having donor atoms like P, S, Cl Soft Acids prefer to bond with Soft Bases.

Hard and Soft Acids

Hard Bases Ligands having highly electronegative donor atoms – F, O, N They prefer complexes with hard acids. Less polarisable Small size Strength of hard bases – F>Cl> Br >O > S> Se>N> P> Soft Bases Donor atom is less electronegative – P,As, Large size High polarisability They prefer complexes with soft acids

Acids H+, Li+, Na+, K+ BBr3,B(CH3)3 Cu+,Ag+, Au+, Hard Acids Borderline Soft Acids H+, Li+, Na+, K+ BBr3,B(CH3)3 Cu+,Ag+, Au+, Be2+, Mg2+, Ca2+ Cd2+,Hg22+, BF3, BCl3, B(OR)3 BH3,Tl+, Tl(CH3)3 Hg2+, Pd2+,Pt2+, Al3+, AlCl3,AlH3 Pt4+ Cr3+,Mn2+, Fe3+, Co3+ Fe2+,Co2+,Ni2+ Cu2+,Zn2+,Rh3+ Ir3+, Ru3+, Os2+

Bases Hard Bases Borderline Soft Bases F-, Cl- Br- H-, I- H2O, OH-,O2- NO2-, N3- , N2 H2S, HS-, S2- ROH, RO-, R2O, CH3CO2- SO32- RSH, RS-, R2S NO3-, ClO4- SCN-, CN-, CO CO32-,SO42-, PO43- S2O32- NH3, RNH2 C6H5NH2 R3P, C6H6

HSAB principle Hard Acids prefer to bond with Hard Bases Soft Acids prefer to bond with Soft Bases. HA and HB are not polarisable so the bond is mostly ionic. SA and SB is highly polarisable so the bond is covalent.

Applications of Hard/Soft Theory 1. Stability of a complex – A complex would be more stable if metal and ligand, both are hard or soft. eg – 1. AgI2- is stable but AgF2- is not. Ag+ + 2I-  AgI2- SA SB Stable complex Ag+ + 2F-  AgF2- SA HB Unstable 2. [CoF6] 3- is more stable than [CoI6] 3- HA-HB HA-SB

2. Feasibility of a reaction A reaction is favourable if the product follows HSAB rule, like combinations. Eg. LiI + CsF  LiF + CsI HA-SB SA-HB HA-HB SA-SB CaS + H 2O  CaO + H 2S HA-SB HB HA-HB SB 3. The Qual Scheme, a series of chemical reactions used to separate and identify the presence of dozens of metal ions, is based largely on the hard and soft properties of the metal ions. The softer metals are precipitated out as chlorides or sulfides, with the harder ions formed as carbonates.

4.Thiocyanate, an ambidentate ligand: SCN- vs. NCS- Thiocyanate (SCN-) is a particularly interesting ligand. It is ambidentate, and can bind to metal ions either through the S or the N. Obviously, it prefers to bind to soft metal ions through the S, and to hard metal ions through the N. This can be seen in the structures of [Au(SCN)2]- and [Fe(NCS)6]3- in the Figure below: Thiocyanate Complexes showing N-bonding in the [Fe(NCS)6]3- complex with the hard Fe(III) ion, and b) S-bonding in the [Au(SCN)2]- complex (CSD: AREKOX) with the soft Au(I) ion

5. Evidence in Nature The chalcophile elements are typically found as sulfides or bonded to Se2- or Te2-. They include: Cd2+, Pb2+, Sb3+, and Bi3+. These are soft Lewis acids. 6.

In general, hard-hard combinations are energetically Hard-soft considerations allow us to make reasonable predictions 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 energetically more favorable than soft-soft But there is more to it… An acid or a base may be hard or soft and 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