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Hard-Soft Acids and Bases: Altering the Cu + /Cu 2+ Equilibrium

Hard-Soft Acids and Bases: Altering the Cu + /Cu 2+ Equilibrium. Objectives: Calculate/predict stability of copper oxidation states Use ligands to change stabilities of oxidation states. HSAB theory: qualitative predictions Redox potentials: quantitative results. Oxidation States.

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Hard-Soft Acids and Bases: Altering the Cu + /Cu 2+ Equilibrium

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  1. Hard-Soft Acids and Bases: Altering the Cu+/Cu2+ Equilibrium • Objectives: • Calculate/predict stability of copper oxidation states • Use ligands to change stabilities of oxidation states HSAB theory: qualitative predictions Redox potentials: quantitative results

  2. Oxidation States • Sum of oxidation states = overall charge on species • Assumes unequal sharing of electrons • more electronegative atom gets all electrons, preferred oxidation state • Examples: • MnO, MnO2, [K+ MnO4-] • What differences are found between metals in different oxidation states? Atomic radius, reactivity Hard/soft, redox potential

  3. Hard vs. Soft Ligands and Metals • Bonding trends of Lewis acids / Lewis bases--electron acceptors / electron donors • Polarizable (soft) vs non-polarizable (hard):

  4. Thermodynamics of Hard/Soft Ligand/Metal Binding HSAB theory: --Hard-hard / soft-soft thermodynamically stronger binding / interaction --Hard-soft / soft-hard thermodynamically weaker binding / interaction Preferential selection of oxidation states by hard or soft ligand set

  5. most stable complexes Kstability = [AB] / [A][B] least stable complexes harder softer

  6. Lewis acids and bases • Hard acids H+, Li+, Na+, K+ , Rb+, Cs+Be2+, Mg2+, Ca2+ , Sr2+, Ba2+BF3, Al 3+, Si 4+, BCl3 , AlCl3Ti4+, Cr3+, Cr2+, Mn2+Sc3+, La3+, Ce4+, Gd3+, Lu3+, Th4+, U4+, Ti4+, Zr4+, Hf4+, VO4+, Cr6+,  Si4+, Sn4+ • Borderline acids Fe2+, Co2+, Ni2+ , Cu2+, Zn2+Rh3+, Ir3+, Ru3+, Os2+R3C+ , Sn2+, Pb2+NO+, Sb3+, Bi3+SO2 • Soft acids Tl+, Cu+, Ag+, Au+, Cd2+Hg2+, Pd2+, Pt2+, M0, RHg+, Hg22+BH3CH2HO+, RO+ • Borderline bases Cl- , Br-NH3, NO2-, N3- SO32-C6H5NH2, pyridine N2 • Soft bases H-, I-H2S, HS-, S2- , RSH, RS-, R2S SCN- (bound through S), CN-, RNC, CO R3P, C2H4, C6H6(RO)3P  • Hard bases F-H2O, OH-, O2-CH3COO- , ROH, RO-, R2ONO3-, ClO4-CO32-, SO42- , PO43-RNH2N2H4

  7. Electrochemical potentials E0 --Related to thermodynamic stability: • G0 = -nFE0 • n = mol e- • F = 96,500 Coulombs / mol e- • E0 = standard reduction potential in volts • G0 = free energy in joules

  8. Electrochemical Potentials Used in Experiment 1: E0 (1) Cu2+ + Iˉ + eˉ  CuI 0.86V (2) Cu2+ + Clˉ + eˉ  CuCl 0.54V (3) I2 + 2eˉ  2Iˉ 0.54V (4) Cu+ (aq) + eˉ  Cu(s) 0.52V (5) Cu2+(aq) + 2eˉ  Cu(s) 0.37V (6) CuCl + eˉ  Cu(s) + Clˉ 0.14V (7) Cu(NH3)42+ + 2eˉ  Cu(s) + 4NH3 -0.12V (8) Cu2+(aq) + eˉ  Cu+ (aq) -0.15V (9) CuI + eˉ  Cu(s) + Iˉ -0.19V (10) Cu(en)22+ + 2eˉ  Cu + 2en -0.50V

  9. Redox Potential Calculation Cu(aq)+2 + 4NH3 Cu(NH3)4+2 (5) Cu2+(aq) + 2eˉ  Cu(s) 0.37V (7) Cu(NH3)42+ + 2eˉ  Cu(s) + 4NH3 -0.12V (5) + (7*) Reduction: Cu2+(aq) + 2eˉ  Cu(s) E0 = +0.37V Oxidation: Cu(s) + 4NH3 Cu(NH3)42+ + 2eˉE0 = +0.12V Net: Cu2+(aq) + 4NH3 Cu(NH3)42+ E0 = +0.49V

  10. Disproportionation --Two identical atoms in same oxidation state exchange one electron --Take on two different oxidation states • 2 Fe4+→ Fe3+ + Fe5+ • 2 H2O2 → 2 H2O + O2 • 2 Cu+ → Cu0 + Cu2+

  11. Summary --Make Cu compounds, force into desired oxidation state with correct H/S ligands --Determine if products agree with HSAB and electrochemical potentials --Determine whether two theories useful in applications

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