Predicting LA-LB reactions

Transcription

1 Predicting LA-LB reactions A chemical reaction is governed by G rxn <0. A qualitative method allows predictions of LA/LB reaction progress. For Bronsted-Lowry Acid-Base interactions the focal point was H +. It is not that clear for the Lewis Model. We need to consider factors that perturb the interaction between electron acceptors and donors. LB LA strength NMe 3 BF 3 > BH 3 PMe 3 or CO BH 3 >BF 3

2 The Pearson LA/LB Hard / Soft Approach Hard Lewis Bases: high EN, low polarizability, hard to oxidize: O, N, F - donors (Cl - is borderline). Soft Lewis Bases: low EN, highly polarizable, easy to oxidize: S, P, I -, Br -, R -, H - donors. Hard Lewis Acids: small, highly charged (high ox. state): H +, alkali metal (M + ) and alkaline earth (M 2+ ) cations, Al 3+, Cr 3+, BF 3. Soft Lewis Acids: large, low oxidation state: Cu +, Ag +, Au +, Tl +, Hg 2+, Pd 2+, Pt 2+, BH 3 Polarizabiity is a major factor in hardness In this model, hard acids like hard bases and soft acids like soft bases.

3 This can be seen in the reactions of numerous compounds. In the Gas Phase HgF 2 (g) + BeI 2 (g) HgI 2 (g) + BeF 2 (g) H rxn = -441kJ In solids LiI (s) + CsF (s) LF (s) + CsI (s) H rxn = -139kJ In the second reaction it is not that CsI is more stable that CsF that drives the reaction it is that LiF is more stable than LiI. More stable compounds are LESS SOLUBLE in aqueous solvents we can predict reaction progress.

4 Uses of the HSAB Approach Metal Cations (M n+ ) that are hard acids (Na +, Ca 2+, Ln 3+, TM 3+ ) prefer hard bases like OR 2 and NR 3 (O and N donor ligands) Note that (M n+ ) like hard bases that stabilize high oxidation states. Metal Cations (M n+ ) that are soft acids (Cu 2+, Cu +, Hg 2+, Tl + ; late TM Pd 2+ and Pt 2+ ) prefer soft bases like SR 2, PR 3, H - ligands. Hence, soft bases stabilize low oxidation state metals. The hard acid-hard base reaction is energetically preferred H + is a hard acid: hard acids show the same order of acidity as the proton

5 Qualitative Analysis An interesting note regarding HSAB phenomenon The solubility of NaX increases from F - to I - (What kind of acid is Na +? What kind of base is water?) The solubility of AgX decreases from F - to I - (What kind of acid is Ag +?) N donating (hard) vs borderline soft

6 More specifically IT is possible to predict solubility based upon HSAB to identify the ions in a given solution. We can classify cations into different groups. They are divided in terms of: insoluble chlorides (Group I), soluble chlorides and very insoluble sulfides (Group II), soluble chlorides and insoluble sulfides (Group III). To distinguish Groups II and III we can control the concentration of S 2- though variation in ph by way of the following equilibria: H 2 S (aq) +H 2 O (l) H 3 O + (aq) + HS - (aq) HS - (aq)+h 2 O (l) H 3 O + (aq) + S 2- (aq) What happens when the ph changes?

7 Classification continued Metals that form soluble sulfides (soft) and insoluble oxides (hard) (Group IV) (these metals have soluble chlorides and sulfides) Metals that form few, if any insoluble compounds (Group V), Group I Group II Group III Group IV Group V AgCl HgS MnS CaCO 3 Na + PbCl 2 CdS FeS SrCO 3 K + Hg 2 Cl 2 CuS CoS BaCO 3 NH 4 + SnS 2 NiS Mg 2+ As 2 S 3 ZnS Sb 2 S 3 Al(OH) 3 Bi 2 S 8 Cr(OH) 3

8 How does HSAB help? What about the silver ions? They dissolve in water and are surrounded by a solvation sphere of water. This is the same for the chloride ions. Is water a hard or soft base/acid? It is a HARD BASE, hence the driving force of this reaction is the preference of the silver ion for the borderline base chloride.

9 Quantitative Measures Stabilization occurs when the donated electrons fill a lower energy M.O. orbital Think of the energy of ionization (I) as a measure of the HOMO energy. Think of the electron affinity (A) as a measure of the LUMO energy Absolute Hardness : η = I A 2

10 E µ = N electronic chemical potential Tendency of the electronic structure of the species to undergo change (escaping tendency of electrons) ( I + A) 2 = Mulliken Electronegativity: Applies to the whole species, Not a particular atom 2 1 E 1 µ η = 2 = 2 N 2 N This is the ABSOLUTE HARDNESS (curvature of the slope of chemical potential) The fractional number of electrons transferred upon a reaction = N Therefore: ( ) N 2( ) ( ) ( ) µ a µ b χa χb = = η + η η + η a b a b The difference in electronegativity drives the electrontransfer And the sum of the hardness parameters measures inhibition Of electron transfer