Chem 241 - UMass Amherstpeople.chem.umass.edu/cjoseph/chem241/lecture15.pdf · 2010-03-02 ·...

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UMass Amherst Biochemistry Teaching Initiative Chem 241 Lecture 15 2 6 6 6 6 6 - ...

Transcript of Chem 241 - UMass Amherstpeople.chem.umass.edu/cjoseph/chem241/lecture15.pdf · 2010-03-02 ·...

Page 1: Chem 241 - UMass Amherstpeople.chem.umass.edu/cjoseph/chem241/lecture15.pdf · 2010-03-02 · Ligand substitution Complexes that lose their ligands easily = Labile ... Elementary

UMass Amherst Biochemistry Teaching Initiative

Chem 241Lecture 15

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Page 2: Chem 241 - UMass Amherstpeople.chem.umass.edu/cjoseph/chem241/lecture15.pdf · 2010-03-02 · Ligand substitution Complexes that lose their ligands easily = Labile ... Elementary

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HomeworkStart reading chapter 20

MOT with both σ and π orbitalsAbsorption LMCT/MLCTFlorescence and Phosphorescence

Recap

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States Ms = ½ for electronS = add all the MsMultiplicity = 2S + 1

S= 0Multiplicity = 1

Singlet

S= 0Multiplicity = 1

Singlet

S= 1Multiplicity = 3

Triplet

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Reactions Ligand substitution

Y + M-X M-Y + X

Kinetics v Thermodynamic

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Formation Constant

M + L ⇋ ML

ML + L ⇋ ML2

M + nL ⇋ MLn

Bn = K1K2K3 …

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Dissociation Constant

ML ⇋ M + L

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Trends in formation ConstantsGeneral Trends

K1 > K2 > K3 …

Chelating Effect – Greater stability once coordinatedHigher K formation because of entropy.

Rings5 and 6 membered rings tend to be more stable

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The Irving-William SeriesSummarize stability of complexes based on electrostatic effects and LFSE

Ba2+ < Sr 2+ < Ca 2+ < Mg 2+ < Mn 2+ < Fe 2+ < Co 2+ < Ni 2+ < Cu 2+ < Zn

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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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