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Hard soft acid_base


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Hard soft acid_base

  1. 1. Pearsons Hard Soft [Lewis] Acid BaseThe Hard Soft [Lewis] Acid Base PrincipleIn the nineteen sixties, Ralph Pearson attempted to explain the differential affinity of electronpair donating Lewis bases towards electron pair accepting Lewis acids, ie. Lewis acid/basecomplexation: A + B –> A-Bby classifying Lewis acids and Lewis bases as hard, borderline or soft.According to Pearsons hard soft [Lewis] acid base (HSAB) principle: Hard [Lewis] acids prefer to bind to Hard [Lewis] bases and Soft [Lewis] acids prefer to bind to Soft [Lewis] basesPearsons HSAB Species Pearsons Hard Lewis Acids: Pearsons Borderline Lewis Acids: (1 of 7)1/12/2004 11:51:35 AM
  2. 2. Pearsons Hard Soft [Lewis] Acid Base Pearsons Soft Lewis Acids: Pearsons Hard Lewis Bases: Pearsons Borderline Lewis Bases: Pearsons Soft Lewis Bases: (2 of 7)1/12/2004 11:51:35 AM
  3. 3. Pearsons Hard Soft [Lewis] Acid BaseIn 1968, G. Klopman quantified Pearsons HSAB principle using frontier molecular orbital(FMO) theory. (The Klopman equation is discussed, here. ) Klopman proposed that: Hard [Lewis] Acids bind to Hard [Lewis] Bases to give charge-controlled ionic complexes and Soft [Lewis] Acids bind to Soft [Lewis] Bases to give FMO-controlled covalent complexesCombining Pearsons and Klopmans ideas: • Hard Lewis acids: Atomic centres of small ionic radius and with net positive charge. Species do not contain electron pairs in their valence shells, they have a low electron affinity and are likely to be strongly solvated. High energy LUMO. • Soft Lewis acids: Large radius, low or partial (delta+) positive charge. Electron pairs in their valence shells. Easy to polarise and oxidise. (3 of 7)1/12/2004 11:51:35 AM
  4. 4. Pearsons Hard Soft [Lewis] Acid Base Low energy LUMOs but large magnitude LUMO coefficients. • Hard Lewis bases: Small, highly solvated, electronegative atoms: 3.0-4.0. Species are weakly polarisable and are difficult to oxidise. Low Low energy HOMO. • Soft Lewis bases: Large atoms of intermediate electronegativity: 2.5-3.0. Easy to polarise and oxidise. High energy HOMOs but large magnitude HOMO coefficients. • Borderline species have intermediate properties. • It is not necessary for species to possess all properties.The Pearson approach is very successful when comparing pairs of species: Sodium ion Na+ is harder than the silver ion Ag+ Alkoxide ions, RO–, are harder than thioanions, RS– Copper(II) ion, Cu2+, is harder than copper(I) ion, Cu+ The nitrogen anion end of the ambidentate cyanide ion, CN–, is harder than the carbon anion end, NC–This type of analysis can be very useful. For example, beta-propiolactone is ring opened bynucleophilic Lewis bases, however, attack can occur at two positions: • Harder nucleophiles like alkoxide ion, R-O–, attack the acyl (carbonyl) carbon. • Softer nucleophiles like thioanion, R-S–, attack the 3-position carbon. (4 of 7)1/12/2004 11:51:35 AM
  5. 5. Pearsons Hard Soft [Lewis] Acid Base This graphic is captured from The Chemical Thesaurus, here. There are a number of examples of this type in this reaction chemistry database.Problems...However, there is a problem.While the Pearson-Klopman HSAB model is not "wrong"... it does grossly oversimplify thereaction chemistry. The reason is that no physical parameter correlates with hardness overPearsons chosen set of species.And as a result, the model introduces ambiguities. How similar are: Pearsons hard Lewis acids: [NH4]+ H+ CO2 Cs+ Cu2+ (5 of 7)1/12/2004 11:51:35 AM
  6. 6. Pearsons Hard Soft [Lewis] Acid Base Pearsons soft Lewis bases: H– R2S: R– benzeneEven more importantly, the interesting hard-soft interactions and complexations are simply notdiscussed by Pearsons model. What about: Sodium hydride Lithium aluminium hydride Lead(IV) acetate Methyl lithium Borane-THF Triethyloxonium tetrafluoroborate FerroceneThe one-dimensional hard-borderline-soft continuum or Pearsons analysis actually hasthe effect of blurring much of the rich, linear (predictable) behaviour which can befound in Lewis acid/base reaction chemistry space.The chemogenesis analysis, here, avoids and explains the pitfalls of thePearson approach.The HSAB Papers R.G.Pearson, J.Am.Chem.Soc., 85, 3533-3543, 1963 R.G.Pearson, Science, 151, 172-177, 1966 R.G.Pearson, Chem. Br., 3, 103-107, 1967 R.G.Pearson, J.Chem.Ed., 45, 581-587, 1968 G.Klopman and R.F.Hudson, Theoret. Chim. Acta, 8, 165, 1967 G.Klopman, J.Am.Chem.Soc., 90, 223-234, 1968 (6 of 7)1/12/2004 11:51:35 AM
  7. 7. Pearsons Hard Soft [Lewis] Acid BaseAlso look here.© Mark R. Leach 2003 (7 of 7)1/12/2004 11:51:35 AM