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Ch12 Lecture 3 Redox and Ligand Reactions
I.
Substitution Reactions of Square Planar Complexes
A.
Kinetics
1) Rate = k1[Complex] + k2[Complex][Y]
a) The k2 term is the normal Assoctiative (A) mechanism
b) The k1 term involves solvent association followed by fast exchange with Y
2)
Solvent effects
a) [Solvent] = large and constant
b) k1 = first order in complex only because k’[Solvent] = k1
B.
Evidence for the A or IA Mechanism
1) The presence of Y in the rate law
2) The isolation of several 5-coordinate intermediates Ni(CN)53)
The detailed study of 4-coordinate T—Pt—X + Y
T—Pt—Y reactions
a) Soft Pt2+ should react strongly with soft bases, not well with hard bases
b) Example k(PPh3)/k(CH3OH) = 9 x 108
4)
Effect of Leaving Group is large on rate
a) Softer leaving groups slow down the reaction
b) p back-bonding increases the ligand strength
c) These are the same p bonds that Y must use as it approaches to bond
II.
The trans-Effect in Pt(II) compounds
A.
Ligand Identity determines what ligand is replaced in sq. pl. Pt(II) complexes
1) Ligands trans to certain other ligands are easily replaced
2) 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
B.
Explanations for the trans-Effect
1) 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.
a) The weak bond makes Pt—X a high energy ground state
b) The Ea required to get X to leave is small
c) Doesn’t quite give the correct trans-Effect ligand ordering
2)
Strong p-acceptors remove e- from Pt making association with Y more likely
a) This interaction from T—Pt lowers the energy of the 5-coord. intermediate
b) Ea is lowered and the Pt—X bond is more easily broken
c) dx2-y2, dxz, and dyz can all p-bond in the trigonal bipyramidal transition state
III. Oxidation-Reduction Reactions (Electron Transfer Reactions)
A.
Redox Basics
1) Oxidation = loss of electrons; metal ion becomes more positively charged
2) Reduction = gain of electrons; metal ion becomes less positively charged
3) Countless biological and industrial processes use metal ions to carry out redox
Photosynthesis, destruction of toxins, etc…
B.
The Mechanisms and Their Characteristics
1) The Outer Sphere Electron Transfer Mechanism = electron transfer with no
change of coordination sphere
a) Example: Co(NH3)63+ + Cr(bipy)32+
Co(NH3)62+ + Cr(bipy)33+
oxidant
reductant
reduced
oxidized
b)
The rates of reaction depend on the ability of the electron to “tunnel” through
the ligands from one metal to the other
i. 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-.
ii. Ligands with p or p orbitals good for bonding more easily allow
tunnelling (CN-, F-) than those that don’t (NH3).
c)
The ligands don’t change in Outer Sphere electron transfer, but the M—L
bond distances do
i. High Oxidation # = short bond distance
ii. Low Oxidation # = longer bond distance
d)
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+ + eCo(H2O)63+ + e-
Co(NH3)62+
Co(H2O)62+
Eo = +0.108 V
Eo = +1.108 V
2)
The Inner Sphere Electron Transfer Mechanism = tunneling of an electron through
a bridging ligand.
a) Substitution links the reactants
b) e- transfer
c) Separation of products
d)
3)
This reaction could be followed by ion exchange and UV-Vis
Choosing Mechanisms
a) Very inert metal ions substitute too slowly to allow bridging: [Ru(NH3)6]2+
b) Ligands that are able to bridge are required for the inner sphere mechanism
c) Most metals can undergo both types of reactions, inner-sphere is more likely
if the metal is very labile (Cr2+)
d) Comparison with experimental data of known reactions helps decide
e)
C.
Reducible ligands speed up inner sphere reactions
Conditions for High and Low Oxidation States
1) Hard ligand select for high oxidation states: MnO4- (Mn+7), PtF6 (Pt+6)
2) Soft ligands select for low oxidation states: V(CO)6 (V0)
Soft favors Cu+
Hard favors Cu2+
Hard should
favor Co3+
But Small Do
wins
Soft should
favor Co2+
But Large Do
wins
IV. Ligand Reactions
A.
RCH2
Organic Chemistry often does reactions on complexed ligands
1) Example: Friedel-Crafts Electrophilic Substitution
X
AlCl3
RCH2
X AlCl3
RCH2
XAlCl3
H
2)
B.
RCH2
The Lewis Acid nature of the metal ion creates positively charged carbon atoms to
react with aromatic rings
Organic and Biological Hydrolysis Reactions
1) Hydrolysis = breaking of C—O or C—N bonds in carboxylic acids and amides
(proteins) or the P—O bond in phosphate esters (DNA)
2)
Coordination of the reacting biopolymer to the metal activates the bond to be
cleaved by the Lewis Acid nature of the metal ion
3)
The reaction can proceed with either bound or free OH- in basic conditions
C.
Template Reactions
1) Template: organizes an assembly of atoms, with respect to one or more geometric
loci to achieve a particular linking of atoms
a) Anchor = organizing entity around which the template complex takes shape,
due to geometric requirements. This is often a metal ion.
b) Turn = Flexible entity in need of geometric organization before the desired
linking can occur
2)
Metal complexes make good templates because many metal ions have strict
geometric requirements, and they can often be removed easily after the reaction.
2+
NH
NH
NH2
NH2
NH
NiCl2
H2O
NH2
Ni
NH
O
O
H
H
H2O
NH2
2+
NH
N
Ni
NH
N
NaBH4
H2O
NH HN
Ni
NH HN
CN
-
H2O
NH HN
NH HN