Ch12 Lecture 3 Redox and Ligand Reactions

1. Substitution Reactions of Square Planar Complexes • Kinetics • Rate = k1[Complex] + k2[Complex][Y] • The k2 term is the normal Assoctiative (A) mechanism • The k1 term involves solvent association followed by fast exchange with Y • Solvent effects • [Solvent] = large and constant • k1 = first order in complex only because k’[Solvent] = k1 Ch12 Lecture 3 Redox and Ligand Reactions

2. Evidence for the A or IA Mechanism • The presence of Y in the rate law • The isolation of several 5-coordinate intermediates Ni(CN)5- • The detailed study of 4-coordinate T—Pt—X + Y T—Pt—Y reactions • Soft Pt2+ should react strongly with soft bases, not well with hard bases • Example k(PPh3)/k(CH3OH) = 9 x 108

3. Effect of Leaving Group is large on rate • Softer leaving groups slow down the reaction • p back-bonding increases the ligand strength • These are the same p bonds that Y must use as it approaches to bond

4. The trans-Effect in Pt(II) compounds • Ligand Identity determines what ligand is replaced in sq. pl. Pt(II) complexes • Ligands trans to certain other ligands are easily replaced • The controlling ligands: (p-acceptor best, strong s-donors next) CN- ~ CO > PH3 ~ SH2 > NO2- > I- > Br - > Cl - > NH3 ~ py > OH- > H2O 3) Examples below only show the “new” product Note: In (g) and (h), the lability of the Cl- leaving group controls

5. Explanations for the trans-Effect • trans-Influence = ground state effect where the strong T—Pt sigma bond using the px and dx2-y2 orbitals prevents the trans leaving group Pt—X bond from being strong. • The weak bond makes Pt—X a high energy ground state • The Ea required to get X to leave is small • Doesn’t quite give the correct trans-Effect ligand ordering • Strong p-acceptors remove e- from Pt making association with Y more likely • This interaction from T—Pt lowers the energy of the 5-coord. intermediate • Ea is lowered and the Pt—X bond is more easily broken • dx2-y2, dxz, and dyz can all p-bond in the trigonal bipyramidal transition state

6. Oxidation-Reduction Reactions (Electron Transfer Reactions) • Redox Basics • Oxidation = loss of electrons; metal ion becomes more positively charged • Reduction = gain of electrons; metal ion becomes less positively charged • Countless biological and industrial processes use metal ions to carry out redox Photosynthesis, destruction of toxins, etc… • The Mechanisms and Their Characteristics • The Outer Sphere Electron Transfer Mechanism = electron transfer with no change of coordination sphere • Example: Co(NH3)63+ + Cr(bipy)32+ Co(NH3)62+ + Cr(bipy)33+ oxidant reductant reduced oxidized

7. b) The rates of reaction depend on the ability of the electron to “tunnel” through the ligands from one metal to the other • Tunneling = moving through an energy barrier (the ligands) that is normally too high to allow the electron to pass through. This is a quantum mechanical process having to do with the wave nature of e-. • Ligands with p or p orbitals good for bonding more easily allow tunnelling (CN-, F-) than those that don’t (NH3). • The ligands don’t change in Outer Sphere electron transfer, but the M—L bond distances do • High Oxidation # = short bond distance • Low Oxidation # = longer bond distance • The stronger the ligand field, the less favored reduction is (or more favored oxidation is), because more energy is gained by losing high energy eg* electrons (NH3 > H2O) Co(NH3)63+ + e- Co(NH3)62+ Eo = +0.108 V Co(H2O)63+ + e- Co(H2O)62+ Eo = +1.108 V

8. The Inner Sphere Electron Transfer Mechanism = tunneling of an electron through a bridging ligand. • Substitution links the reactants • e- transfer • Separation of products • This reaction could be followed by ion exchange and UV-Vis • Choosing Mechanisms • Very inert metal ions substitute too slowly to allow bridging: [Ru(NH3)6]2+ • Ligands that are able to bridge are required for the inner sphere mechanism • Most metals can undergo both types of reactions, inner-sphere is more likely if the metal is very labile (Cr2+) • Comparison with experimental data of known reactions helps decide

9. Reducible ligands speed up inner sphere reactions • Conditions for High and Low Oxidation States • Hard ligand select for high oxidation states: MnO4- (Mn+7), PtF6 (Pt+6) • Soft ligands select for low oxidation states: V(CO)6 (V0)

10. Soft favors Cu+ Hard favors Cu2+ Hard should favor Co3+ But Small Do wins Soft should favor Co2+ But Large Do wins

11. Ligand Reactions • Organic Chemistry often does reactions on complexed ligands • Example: Friedel-Crafts Electrophilic Substitution • The Lewis Acid nature of the metal ion creates positively charged carbon atoms to react with aromatic rings • Organic and Biological Hydrolysis Reactions • Hydrolysis = breaking of C—O or C—N bonds in carboxylic acids and amides (proteins) or the P—O bond in phosphate esters (DNA) • Coordination of the reacting biopolymer to the metal activates the bond to be cleaved by the Lewis Acid nature of the metal ion • The reaction can proceed with either bound or free OH- in basic conditions

13. Template Reactions • Template: organizes an assembly of atoms, with respect to one or more geometric loci to achieve a particular linking of atoms • Anchor = organizing entity around which the template complex takes shape, due to geometric requirements. This is often a metal ion. • Turn = Flexible entity in need of geometric organization before the desired linking can occur • Metal complexes make good templates because many metal ions have strict geometric requirements, and they can often be removed easily after the reaction.