Transcript soft fig
Hard and soft acids and bases Classical concepts Arrhenius: • acids form hydrogen ions H+ (hydronium, oxonium H3O+) in aqueous solution • bases form hydroxide ions OH- in aqueous solution • acid + base salt + water e.g. HNO3 + KOH KNO3 + H2O Brønsted-Lowry: • acids tend to lose H+ • bases tend to gain H+ • acid 1 + base 1 base 1 + acid 2 (conjugate pairs) H3O+ + NO2- H2O + HNO2 NH4+ + NH2- NH3 + NH3 In any solvent, the reaction always favors the formation of the weaker acids or bases The Lewis concept is more general and can be interpreted in terms of MO’s Acids and bases (the Lewis concept) A base is an electron-pair donor An acid is an electron-pair acceptor acid adduct base Lewis acid-base adducts involving metal ions are called coordination compounds (or complexes) In most acid-base reactions HOMO-LUMO combinations lead to new HOMO-LUMO of the product But remember that there must be useful overlap (same symmetry) and similar energies to form new bonding and antibonding orbitals Hard and soft acids and bases Hard acids or bases are small and non-polarizable Soft acids and bases are larger and more polarizable Halide 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 p-bonding 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. Higher oxidation states of elements to the right of transition metals have more class b character since 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 hard low 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* orbitals available for π-bonding react best with class (b) soft metals Tendency to complex with hard metal ions N >> P > As > Sb O >> S > Se > Te F > Cl > Br > I Tendency to complex with soft metal ions N << P > As > Sb O << S > Se ~ Te F < Cl < Br < I The hard-soft distinction is linked to polarizability, the degree to which a molecule or 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 electrons Shield 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 energetically more favorable than soft-soft 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 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 related With lower solubility, color and short interionic distances, whereas hard-hard interactions result in colorless and highly soluble compounds Quantitative measurements IA 2 Absolute hardness (Pearson) 1 Softness IA 2 Mulliken’s absolute electronegativity (Pearson) EHOMO = -I ELUMO = -A Pearson’s Hard and Soft Acids and Bases Most of the hard-soft distinction depends on polarizability. Hard acids and bases are small, compact, and nonpolarizable. Most metal ions are hard acids (class a ions, Fig. 6-11). Hardness of a metal ion generally increases with its charge. Soft acids are generally large and polarizable (class b ions). Glead to softer behavior. Soft acids can often be characterized as havenerally, more electrons and larger sizes ing d electrons/orbitals for p bnding. Tl(I) versus Tl(III) – Tl(III) has more class be character even though the charge on the metal is greater. Why? Pearson’s Principle: Hard Lewis acids prefer to bind to hard Lewis bases; soft Lewis acids prefer to bind to soft Lewis bases Class (a)– hard acids Class (b)– soft acids Hard and Soft Acids and Bases (HSAB) Let A be a Lewis acid, and B a base Measure log K for the reaction A + B AB If for B = halide, the order of log K is I– < Br– < Cl– < F– then A is called a hard acid If for B = halide, the order of log K is I– > Br– > Cl– > F– then A is called a soft acid Hard metal ions form their most stable complexes with Hard Bases Hard Bases: contain the smaller electronegative atoms, especially O, N, F and Cl. The bonding between a Hard Lewis Acid and a Hard Lewis Base is predominantly ionic Soft metal ions form their most stable complexes with Soft Bases Soft Bases: contain the larger, more polarisable and less electronegative atoms, especially S, Se, P, C and As. The bonding between a Soft Lewis Acid and a Soft Lewis Base is predominantly covalent Quantitative measure Absolute hardness = (I –A)/2 Mulliken’s definition of electronegativity = (I + A)/2 Softness = 1/ Reactions favor hardness matches Hard=small, compact charge, and nonpolarizable. M+3, O-2 Soft=large, polarizable. M0, S2- Acid and Base Strength Binary hydrogen compounds The acidity increases across a period (the electronegativity also increases). HF>H2O>NH3 Explanation: The negative charge of the conjugate base is spread out over more lone pairs (electrons) as the base gets larger. The larger the number of lone pairs, the lower the attraction for protons. Acid-Base Strength, Inductive Effects • Electronegativity effects can can change the acidity and basicity. – PF3 is a weaker base than PH3 due to the electrons being drawn toward the electronegative fluorines. NMe3>NHMe2>NH2Me>NH3 (base strength) Alkyl groups tend to increase the electron density on the center atom. Electron-contributing and electron-withdrawing capabilities of the ligands need to be considered. Acid-Base Strength, Oxyacids • HClO4>HClO3>HClO2>HClO – pKa 9-7n • n = number of nonhydrogenated oxygen atoms. Explanation: Nonhydrogenated oxygens draw electron density from the central atom. The central atom, in turn, draws electron density away from he hydrogenated oxygen. The net result is a weaker O-H bnd. Other explanation: The charge of the conjugate base is spread over all the nonhydrogenated oxygens. Stability of the conjugate base increases with the number of these oxygens producing a weaker base. Acidity of Cations in Aqueous Solutions • • • In general, stronger acids form from metal ions with larger charges and smaller radii. – [Fe(H2O)6]3+ + H2O [Fe(H2O)5OH]2+ + H3O+ Solubility of the metal hydroxide is also a measure of cation acidity. The stronger the cation acid, the less soluble the hydroxide. – 3+ metal ions form hydroxides that precipitate. • Fe+3(aq) + 3H2O(l) Fe(OH)3(s) + 3H+(aq) – 2+ d-block ions (and Mg+2) from hydroxide precipitates in neutral or slightly acidic solutions – Alkali (e.g. Na+) and alkaline earth metals produce no pH changes (spectators). The free metal cation is no longer detectable for highly-charged species. – UO2+ and CrO42- Steric Effects • Repulsions by bulky groups make reactions less favorable for adduct formation. • Different types of strain (Brown): – F (front) – bulky groups interfere directly with the approach. – B (back) – bulky groups interfere with each other when they bend away from the other molecule forming the adduct. – I (internal) – electronic differences within similar molecules. Examples Solvation • In aqueous solution the basicities have the order – Me2NH>MeNH2>Me3N>NH3 Why? – The reduced solvation of their protonated cations. • Solvation energies; RNH3+>R2NH2+>R3NH+ for the reaction RnH4-nN+(g) + H2O RnH4-nN+(aq). Solvation is dependent upon the number of hydrogen atoms available to hydrogen bond. – Competition between induction and solvation produce the observed order for the amines. Nonaqueous Solvents • Any acid stronger than H3O+ reacts completely with H2O to form H3O+. The is called the leveling effect; acids or bases are brought down to the limiting conjugate acid or base of the solvent. – HCl, HBr, HClO4, and HNO3 all have the same strength in water. • Likewise, the strongest base in water is OH-. – Water can differentiate weak acids and bases. Nonaqueous Solvents • Acetic acid can differentiate the strengths of many strong acids. – H2SO4 + HOAc H2OAc+ + HSO4– In acetic acids, HClO4>HCl>H2SO4>HNO3 • Inert solvents allow a broad range of acids and bases to be differentiated according to strength. The do not form acid or base species readily. – Figure 6-17. Hard and Soft Acids and Bases.