UMass Amherst Biochemistry Teaching Initiative

Chem 241Lecture 16

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Announcement March 10 LGRT 0323

Formation ConstantEquilibriumsIrving-Williams series

Recap

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Ligand substitution Complexes that lose their ligands easily = LabileComplexes that keep their ligands = Nonlabile or Inert

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Mechanism Rate Law links the rate of the reaction to the concentrations.

xA + yB C r = k[A]x [B]y

Elementary step represents the reaction at the molecular level not the overall reaction.

2 NO2 + F2 2NO2F

NO2 + F2 NO2F + FNO2 + F NO2F

Rate-Determing Step slowest elementary step

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Mechanism Dissociative Mechanism is a reaction in which an intermediate of reduced coordination number is formed by the departure of the leaving group.

MLnX MLn + XMLn + Y MLnY

Rate Determining Step:Breaking the bond of theLeaving group

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Dissociative Mechanism

• The geometry of the end product can be different• Reaction order is 1

Happens if L can stabilize square planar, L is strong π-acceptor and/or strong σ-donor.

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Dissociative Mechanism

The first step is rate determiningk2 >> k1

rate = k1 [MXn]

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Mechanism Associative Mechanism is a reaction in which an intermediate of higher coordination number is formed.

MLnX + Y MLnXYMLnXY MLnY + X

Rate Determining Step:Ligand attack

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Associative Mechanism

• The geometry does not get scrambled • Reaction order is 2

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Dissociative Mechanism

The first step is rate determiningk2 >> k1

rate = k1 [MXn][Y]

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Mechanism Interchange Mechanism takes place in one step.

MLnX + Y X···MLn···YX···MLn···Y MLnY + X

Often we don’t have a pure associative or dissociative mechanism – instead the reaction involves concerted bond breaking and formation.

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Square Planar Platinum is an archetype

Nucleophilicity parameter

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Trans Effect

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Trans Effect Is defined as the labilization of ligands trans to certain other ligands.

CN-, CO, NO, C2H2 > PR3, H- > CH3- , C6H5

- , SC(NH2)2, SR2 > SO3H- > NO2

-, I-, SCN- > Br- > Cl- > py > RNH2, NH3 > OH- > H2O

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Trans Effect Trans influence is the extent to which the ligand T weakens the bond trans to iteself in the ground state

T σ-donor: OH- < NH3 < Cl- < Br- < CN-, CH3- < I- < SCN- < PR3

T π-acceptor: Br- < I- < NCS- < NO2- < CN- < CO, C2H2

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Steric Effects Steric crowding usually hinders associative attacks.

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Tolman Cone AngleApproximating the ligand as a cone.

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IsomerizationGo by a dissociative mechanism:Bailer twistRay- Dust twist

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Redox ReactionsReduction – Electron Gain Oxidation – Electron Loss

Reducing Agent – Species that supplies electronsOxidizing Agent – Species that removed electrons

Groups 1 and 2 will do oxidation states of +1 and +2, respectively. While some d metals like Os can go from 0 to +8

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Redox Half ReactionsReduction – Electron Gain

Ag+(aq) + e- Ag(s)

Oxidation – Electron LossCu(s) Cu2+(aq) + 2e-

Ag+(aq) + e- + Cu(s) Cu2+(aq) + 2e- + Ag(s)2Ag+(aq) + 2e- + Cu(s) Cu2+(aq) + 2e- + 2Ag(s)

2Ag+(aq) + Cu(s) Cu2+(aq) + 2Ag(s)

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Redox Mechanisms

[CoCl(NH3)5]2+ + [Cr(H2O)6]2+ → [Co(NH3)5(H2O)]2+ +[CrCl(H2O)5]2+

[M(H2O)6]n+1 + [M(H2O)6]n[M(H2O)6]n + [M(H2O)6]n+1

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Photochemical Reactions

[Cr(NH3)6]3+ + H2O + hv [Cr(NH3)5(H2O) ]3+ + NH3

Sometimes not so simple:[CoIIICl(NH3)5]3+ + H2O + hv [CoII(NH3)5(H2O) ]2+ + Cl·[CoII(NH3)5(H2O) ]2+ + Cl· [CoIII(NH3)5(H2O) ]3+ + Cl-

Quantum Yeild (Φ) – is the number of defined events, in terms of reactant or product, that occur per photon ansorbed by the system.

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Standard Cell PotenialAg+(aq) + e- Ag(s) Eo = 0.799 V

Cu(s) Cu2+(aq) + 2e- Eo = -0.337 V

2Ag+(aq) + Cu(s) Cu2+(aq) + 2Ag(s) Eo = 0.462

ΔGo = -nFEo F= 96.48 kC mol-1

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