1 W. B. Jensen, The Lewis Acid-Base Concepts (1980) Most of this chapter is concerned with the Lewis definition, its more recent explanation in terms of molecular orbitals, & its application to inorganic chemistry. 2 6-1-1 History 3 6-2 Major Acid-Base Concepts 6-2-1 Arrhenius Concept Arrhenius received 1903 Nobel Prize in chemistry for this theory, Arrhenius acids form hydrogen ions (now frequently called hydronium or oxonium + ions, H3O ) in aqueous solution, Arrhenius bases form hydroxide ions in aqueous solution. 4 This explanation works well in aqueous solution, but it is inadequate for nonaqueous solutions & for gas & solid phase reactions in which H+ & OH- may not exist. Definition by Br∅nsted & Lewis are more appro- priate for general rule. 5 6-2-2 Br∅nsted-Lowry concept In 1923, Br∅nsted-Lowry defined an acid as a species with a tendency to lose a hydrogen ion & a base as a species with a tendency to gain a hydrogen ion. This definition expanded the Arrhenius list of acids & bases to include the gaseous HCl & NH3. 6 This definition also introduced the concept of conjugate acids & bases, differing only in the presence or absence of a proton, & described all reactions as occurring between a stronger acid & base to form a weaker acid & base. 7 Na + NH3 8 6-2-3 Solvent System Concept The solvent system definition applies to any solvent that can dissociate into a cation & an anion (autodissociation), where the cation resulting from autodissociation is the acid & the anion is the base. Solutes that increase the conc of the cation of the solvent are considered acids & solutes that increase the conc of the anion are considered bases. + - 2H2O ' H3O + OH Solvent: H2O + - H2SO4 + H2O ' H3O + HSO4 Solute: H2SO4 (acid) + - 2BrF3 ' BrF2 + BrF4 Solvent: BrF3 + - SbF5 + BrF3 ' BrF2 + SbF6 Solute: SbF5 (acid) - - - F + BrF3 ' BrF4 Solute: F (base) 9 Properties of Solvents 10 6-2-4 Lewis concept A base as an electron-pair donor and an acid as an electron-pair acceptor. Adduct is a combination of a Lewis acid & base (metal-containing adducts are called coordination compounds). acid base adduct 11 Because fluorine is the most electronegative element, the boron atom in BF3 is quite positive, & the boron is frequently described as electron-deficient. The lone pair in the HOMO of NH3 combine with the empty LUMO of BF3, which has very large, empty orbital lobes on boron, to form the adduct. LUMO HOMO A + :B → A-B Acid adduct Base 12 LUMO HOMO empty orbital lone pair lobes on boron on nitrogen Note the change of the geometry of BF3. 13 14 b.p. : BF3 (-99.9 °C); diethyl ether : 34.5 °C adduct : 125 - 126 °C. The formation of the adduct raises the boiling point enormously, a common result of such reactions. 15 6-2-5 Frontier orbitals and acid-base reactions Frontier orbitals : those at the occupied-unoccupied frontier (HOMO - LUMO) + + NH3 + H NH4 acid-base reaction The combination of the HOMO of the base (NH3) & the LUMO of the acid (H+). 16 In most acid-base reactions, a HOMO-LUMO combination forms new HOMO & LUMO orbitals of the product. 17 Td C3v 18 (I) A-B dismatch in energy (A < < B) 2+ - 2H2O + Ca Ca + 2OH + H2 (water as oxidant) (II) A-C match in energy (A < C) - - nH2O + Cl [Cl(H2O)n] (water as acid- solvation of anion) 19 - H2OCaCl (III) A-D match in energy (A > D) 2+ 2+ 6H2O + Mg [Mg(H2O)6] (water as base – solvation of cation) (IV) A-E dismatch in energy (A > > E) - + 2H2O + 2F2 4F + 4H + O2 (water as reductant) 20 Lewis definition of acid & base in terms of frontier orbitals : A base has an electron pair in a HOMO of suitable symmetry to interact with the LUMO of the acid. The better the energy match between the base’s HOMO & the acid’s LUMO, the stronger the interaction. 21 6-2-6 Hydrogen bonding Symmetrical H-bonding: FHF- Note that the LUMO of HF is mainly located on H-atom side. F- FHF- HF 22 unsymmetrical H-bonding: BHA There are three possible cases, as shown in Figure 6-8. 23 Figure 6.8 (a) Poor match of HOMO and LUMO. LUMO No energy gain with adduct formation. HOMO Little or no H-bonding. For example: H2O H2O···H-CH3 CH4 24 Figure 6-8 (b) Good HOMO (base) and LUMO (acid) match Both occupied orbitals are lowered in energy, with net energy gain. If the B HOMO is higher than the A LUMO, as in this figure, the H-A portion is stronger. HOMO If the B HOMO is lower LUMO than the A LUMO, the H-B portion is stronger. It is closer so that the HOMO of the adduct B…H-A contains more A. 25 Figure 6-8 (c) The HOMO-LUMO energy match is so poor that H+ transfer occurs. More energy gain than loss, so that proton prefers transferring to B (base). Energy loss Energy gain 26 487 507 (6 eq) Orange brown 558 Yellow Violet (1) 475 (MLCT) 13.08 13.09 UV-vis spectra of 1 in MeCN solution and the 13.10 addition of 1 equiv of anions and 6 equiv TBAF. 13.10 13.45 H-bond 13.88 vanished 1 H-NMR spectra of 1 in DMSO-d6 in the Ye et al, Inorg. Chem. 2007, 46, 6427. absence and presence of 1 equiv of anions. 6-2-7 Electronic spectra (including Charge Transfer) blue-shift 28 CT bands : 230-400 nm 520 nm D-A bands : ~500 nm 520 nm 500 nm 450 nm I -;360 nm 3 29 Charge-transfer spectra Charge-transfer absorption a strong interaction between a donor solvent and a halogen molecule, X2, leads to the formation of a complex in which an excited state (primarily of X2 character, LUMO) can accept electrons from a HOMO (primarily of solvent character) on absorption of light of suitable energy : 30 . + - X2 donor → [donor ][X2 ] The absorption band, known as a charge-transfer band, can be very intense; it is responsible for the vivid colors of some of the halogens in donor solvents. 31 e- CoIII−X- CoII···X CT Charge-transfer bands Blue-shifted from Br- to F- Hight of X- HOMO: Br- > Cl- > F- Ease of oxidation: Br- > Cl- > F- d-d transition of Co3+(d6) 1kK = 1000 cm-1 CT usually has larger absorption coefficient than d-d transition.32 6-2-8 Receptor-Guest Interactions Also called inclusion complex (C60 + C60H24) A shortest C…C distance of 3.128 Å (rvw = 1.70 Å) 33 Schematic illustration of the attractive electrostatic interaction between the σ framework & the π electron density in an offset π-stacked & in a T-shaped geometry because of decreased π⋅⋅⋅π repulsion 6-3 Hard and Soft Acids and Bases 1. Relative solubility of halides Mercury(I) halides have a similar trend (Ksp : Hg2F2 > Hg2I2). 35 2. Coordination of thiocyanate (SCN-) to metals : 2- 2- [Hg(SCN)4] vs [Zn(NCS)4] 3. Equilibrium constants of exchange reactions. + + [CH3Hg(H2O)] + HCl ⇔ CH3HgCl + H3O K = 1.8 x 1012 + + [CH3Hg(H2O)] + HF ⇔ CH3HgF + H3O K = 4.5 x 10-2 36 37 Linkage Isomerism 2- 2- 2- [Co(NCS)4] , [Hg(SCN)4] , [Pt(SCN)4] 2- [Pd(SCN)4] at RT (solid) 2- [Pd(NCS)4] at HT (solid) or in solution [Pd(bpy)(SCN)2] : two isomers obtained. Factors influencing Solubility : 1. Solvation of the ions (exothermic) 2. Ag-X interactions [hard and soft acids and bases (HSAB)] (endothermic) 39 • Hard : small and nonpolarizable (polarizability : charge separation) Soft : larger and more polarizable Soft-soft > hard-hard > hard-soft • Color : depends on the difference in energy between occupied and unoccupied orbitals. AgI yellow; AgBr slightly yellow; AgCl, AgF white. (LMCT) 40 Rules by Fajan in 1923 (Third edition) 1. For a given cation (smaller EN), covalent character increases with increase in size of the anion (smaller EN). 2. For a given anion (larger EN), covalent character increases with decrease in size of the cation (larger EN). 3. Covalent character increases with increasing charge on either ion. 4. Covalent character is greater for cations with nonnoble gas electronic configurations. 41 (S-S) (H-H) 42 Solubility: MgCO3 > CaCO3 > SrCO3 > BaCO3 Fajan‘s Rule 2 predict the reverse of this order. Mg2+ (small, with higher charge density) – strongly solvated with water (HSAB) Ba2+ (large, with smaller charge density) The difference appears to lie in the aquation of the metal ions. Mg2+ attracts water molecules much more strongly than the others, with Ba2+ the least strongly solvated. 43 Ahrland, Chatt, and Davies divided the metal ions into two classes : The solubility of class (b) metal halides : F- > Cl- > Br- > I- The order is reverse for class (a) metal halides. Class (b) metal ions have a larger enthalpy of reaction with P-donors than with N-donors. This is again reverse for class (a) metal ions. 44 Class (a) Class (b) Borderline elements, whose behavior depends on their O.S. & the donor. 45 Class (b) ions have d electrons available for π bonding (back bonding). Donor molecules or ions that have the most favorable enthalpies of reactions with class (b) metals are those that are readily polarizable and have vacant d or π* orbitals available for π bonding. 46 6-3-1 Theory of Hard and Soft Acids and Bases Pearson designated the class (a) ions hard acids and class (b) ions soft acids. Reactions are more favorable for hard-hard and soft-soft interactions than for a mix of hard and soft in the reactants.