Hard-Soft Acids and Bases: Altering the Cu+/Cu2+ Equilibrium

Objectives:

(1) Calculate/predict stability of copper oxidation states

(2) Use ligands to change stabilities of oxidation states

HSAB theory: qualitative predictions

Redox potentials: quantitative results

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

Hard vs. Soft Ligands and Metals

• Bonding trends of Lewis acids / Lewis bases

--electron acceptors / electron donors

•Polarizable (soft) vs non-polarizable (hard):

Thermodynamics of Hard/Soft Ligand/Metal Binding

--Hard-hard / soft-soft thermodynamically stronger binding / interaction

--Hard-soft / soft-hard thermodynamically weaker binding / interaction

HSAB theory:

Preferential selection of oxidation states by hard or soft ligand set

Kstability = [AB] / [A][B]

softerharder

most stable complexes

least stable complexes

Lewis acids and bases

• Hard acids H+, Li+, Na+, K+ , Rb+, Cs+ Be2+, Mg2+, Ca2+ , Sr2+, Ba2+ BF3, Al 3+, Si 4+, BCl3 , AlCl3 Ti4+, 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+, Hg2

2+ BH3 CH2 HO+, RO+

• Hard bases F- H2O, OH-, O2- CH3COO- , ROH, RO-, R2O NO3-, ClO4- CO3

2-, SO42- , PO4

3- RNH2 N2H4

  • 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 

G0 = -nFE0

n = mol e-

F = 96,500 Coulombs / mol e-

E0 = standard reduction potential in volts

G0 = free energy in joules

Electrochemical potentials E0

--Related to thermodynamic stability:

(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

Electrochemical Potentials Used in Experiment 1:

E0

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

Reduction: Cu2+(aq) + 2eˉ Cu(s) E0 = +0.37V

Oxidation: Cu(s) + 4NH3 Cu(NH3)42+ + 2eˉ E0 = +0.12V

Net: Cu2+(aq) + 4NH3 Cu(NH3)42+ E0 = +0.49V

(5) + (7*)

Disproportionation

• 2 Fe4+ → Fe3+ + Fe5+

• 2 H2O2 → 2 H2O + O2

--Two identical atoms in same oxidation state exchange one electron

--Take on two different oxidation states

2 Cu+ → Cu0 + Cu2+

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