Inorganic chemistry B Sc III Hard acid and

Inorganic chemistry B. Sc III Hard acid and soft acid and bases

Hard and Soft Acids and Bases • 1965 - Ralph Pearson introduced the hardsoft-acid-base (HSAB) principle. • Hard acids prefer to coordinate the hard bases and soft acids to soft bases” • This very simple concept was used by Pearson to rationalize a variety of chemical information. • 1983 – the qualitative definition of HSAB was converted to a quantitative one by using the idea of polarizability. • A less polarizable atom or ion is “hard” and a more easily polarized atom or ion is “soft”

• Hard acid: • High positive charge • Small size • Not easily polarizable • Hard base: • Low polarizability • High electronegativity • Not easily oxidized

• Soft acid: • Low positive charge • Large size; easily oxidized • Highly polarizable • Soft base: • • High polarizability Diffuse donor orbital Low electronegativity Easily oxidized

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: Ag. F > Ag. Cl > Ag. Br >Ag. I But…… Li. Br > Li. Cl > Li. I > Li. F

Chatt’s explanation. Class (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 6 s electrons outside the 5 d 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

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

Application of HSAB principle

Quantitative measurements Absolute hardness (Pearson) Mulliken’s absolute electronegativity (Pearson) EHOMO = -I ELUMO = -A Softness

Electronic Factors HSAB Concepts • Using HSAB guidelines, reactions between acids and bases can be often be predicted successfully (though not always) Q: Is OH- or S 2 - more likely to form an insoluble salt with a +3 transition metal ion? A: The harder species will bind more strongly. Between OH- or S 2 -, OH- is the harder species.

Electronic Factors STABILITY OF COMPLEX Q: Why is Ag. I(s) very water-insoluble, but Li. I very water-soluble? A: Ag. I is a soft acid-soft base combination, while Li. I is hard-soft. The interaction between Li+ and I- ions is not strong. Ag. I(s) + H 2 O(l) essentially no reaction Li. I(s) + H 2 O(l) Li+(aq) + I-(aq)

Qualitative Analysis • In the separation of the group cations carried out this year, HSAB rules were used to separate classes of ions based on different hard and soft interactions soft and 2+ 2+ 3+ 3+ • Group II: Hg , Cd , Cu , Sn , Sb , Bi borderline acids • Group III: Mn 2+, Fe 2+, Cu 2+, Ni 2+, Zn 2+, Al 3+, Cr 2+borderline • Group IV: Ca 2+, Sr 2+, Ba 2+, K+, NH 4+ hard acids

Separation of Cations • The soft and borderline cations are separated through reaction with the soft base sulfide, S 2 -. Group II sulfides are less soluble than group III, so in order to selectively remove group II ions, a low p. H is used: H 2 S(g) D 2 H+(aq) + S 2 -(aq) • Even at low S 2 - concentrations, the group II ions precipitate (stronger interactions with the soft base, S 2 -) • Raising the p. H increases the S 2 - concentration, which allows the precipitation of group III ions • The group IV are then precipitated as hydroxides. These cations are harder and prefer the hard base OH-.

Ambidentate Bases • SCN- (thiocyanate) can interact through either its S or N atom with Lewis acids. It can donate an electron pair through more than one atom. • Interaction will be through the S-atom with a soft acid, or through the N-atom when interacting with hard acids. • Cr(III) interacts as Cr-NCS, while Pt(II) does so as Pt-SCN

Electronic Factors Inductive Effects • Electron donating substituents enhance base strength and electron-withdrawing groups enhance electron acceptor (acid) strength gas-phase base strengths PMe 3 stronger base than PH 3 NMe 3 > NHMe 2 > NH 2 Me > NH 3 strongest base weakest base This plays a role in bond lengths also Me = methyl; alkyl, aryl groups are electron donating; F, CF 3, CN, etc. are e- withdrawing

Structural Factors Structural Rearrangement • In some cases, a center must adjust its hybridization in order to accommodate the formation of a new bond sp 2 opposite order to what is expected for inductive effect sp 3 • Order of Lewis acid strength for BX 3 (X = halides) is BF 3 < BCl 3 < BBr 3 • This is due to better p-orbital overlap in BF 3 than in BCl 3, which is better than BBr 3 (B-F bonds are shortest). Thus more energy is needed to change from the sp 2 -hybridized form of BF 3.

• Predict which way the following reactions will go. • HI + Na. F HF + Na. I R • • Al. I 3 + 3 Na. F Al. F 3 + 3 Na. I R • Ca. S + H 2 O Ca. O + H 2 S R • Ti. F 4 + 2 Ti. I 2 Ti. I 4 + 2 Ti. F 2 L • Co. F 2 + Hg. Br 2 Co. Br 2 + Hg. F 2 L • Hg. O + H 2 S Hg. S + H 2 O R

Predict which way the following reactions will go. HI + Na. F -----HF + Na. I R Al. I 3 + 3 Na. F Al. F 3 + 3 Na. I R Ca. S + H 2 O Ca. O + H 2 S R Ti. F 4 + 2 Ti. I 2 Ti. I 4 + 2 Ti. F 2 L Co. F 2 + Hg. Br 2 Co. Br 2 + Hg. F 2 L Hg. O + H 2 S Hg. S + H 2 O R