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Reactions, Kinetics and Mechanisms

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1 Reactions, Kinetics and Mechanisms
Square Planar Complexes. Consider two complexes, [PtCl4]-2 and [Pt(NH3)4]+2. If the first were to react with two moles of NH3 and the second with two moles of Cl-, would these reactions yield the same product, [PtCl2(NH3)2]? So, would predict that starting from either complex, should get an 50:50 mixture of cis and trans isomers. In fact, [PtCl4]-2 produces only the cis isomer and [Pt(NH3)4]+2 produces only the trans isomer. H3N Cl Pt Cl NH3 trans cis [ ] [ ] Cl Cl Pt Cl Cl +NH3 -Cl- +NH3 -Cl- Cl Cl Pt Cl NH3 Cl NH3 Pt [ ] H3N Cl Pt Cl NH3 [ ] trans cis H3N NH3 +2 Pt H3N NH3 H3N NH3 + Pt H3N Cl -NH3 +Cl- -NH3 +Cl- Cl NH3 Pt

2 [ ] [ ] [ ] [ ] [ ] [ ] [ ] Trans Effect
Note that in the second step, substitution always occurred trans to one of the chlorines. DEFINE: Trans Effect is the labilization of ligands trans to other, trans-directing ligands: By ordering the sequence of addition of substituents, can use the trans effect to produce a desired isomer. CN- ~ CO > NO2- ~ SCN- ~ I- > Br- > Cl- > py > NH3 > OH- > H2O [ ] [ ] [ ] [ ] Cl Cl Pt Cl Cl + NH Br py Cl Cl Pt Cl NH3 Cl Br Pt Cl NH3 Cl Br Pt py NH3 +py [ ] [ ] [ ] Cl Cl Pt Cl py + Br NH3 Cl Br -1 Pt Cl py Cl Br Pt H3N py

3 Explanations for the Trans Effect
Polarization Theory (Grinberg c. 1935). in a completely symmetrical complex, such as [PtCl4]-2 the bond dipoles to the various ligands will be identical and will cancel. however, if another, more polarizing ligand (T) is introduced, an additional uncompensated dipole in the metal will be introduced. the induced dipole in the metal will oppose the natural dipole of the ligand trans to T, making this bond weaker. therefore, the more polarizing ligand will be the trans-director. δ- L L ─ M ─ L δ- L L ─ M ─ T δ δ- L M T δ- δ+

4 Explanations for the Trans Effect
Static π-Bonding Theory. two π-bonding ligands will “fight” for the d-orbital electron density of the metal. since there is less overlap between M and L, this bond will be weaker. Therefore, π-bonding ligand will be stronger trans-director. Problem: this doesn’t explains what happens when ligands are not π-bonders, for example, ammonia. Both mechanism might be used to explain the trans effect, and do explain trends, however, these reactions have all been found to be second order. this means that the rate determining step involves both the initial complex and the incoming ligand. Therefore, it is an associative (Sn2) and not a dissociative (Sn1) mechanism as the two theories would suggest. T = stronger π-bonder L M T more electron density [Pt X3T] E → [PtX2ET] rate = [PtX3T] [E]

5 Mechanisms of Redox Reactions
At first glance it might appear that the mechanisms of redox reactions would be trivial. That is, the two metals approach each other, electron transfer takes place, and the metals go on their way. However, these reactions are complicated by the fact that the metals are surrounded by ligands. This results in two types of redox reactions: Outer Sphere in outer sphere reactions, the coordination sheres are not altered. e.g. [Fe(CN)6] [Mo(CN)8]-3 → [Fe(CN)6] [Mo(CN)8]-4 in general, these reaction rates are faster because no bonds are being broken.

6 Mechanisms of Redox Reactions
Inner Sphere. here the ligand is intimately involved in electron transfer. note that inner sphere reactions involve a physical bridge by a ligand from one species to another during the redox reaction. evidence for direct transfer: if the reaction is run in a solution with radiolabeled Cl-, little or none of the radiolabel is found in the product. Inner sphere reactions are much slower that outer sphere reactions. [Co(NH3)5Cl] [Cr(H2O)6]+2 [(NH3)5Co─Cl─Cr(H2O)5] H2O [(NH3)5Co] [Cr(H2O)5Cl]+2 Frank-Condon Principle: the motion of nuclei is very slow as compared to electrons.

7 Consequences of Inner Sphere Reactions
Can get transfer of ligand; transferable ligand necessary, but actual transfer is not. Note: bridging ligand must have second, bond-forming, lone pair. Rate can be no faster that the exchange rate of the ligand in the absence of the redox reaction. The rate determining step will be zero order in one of the reactants. if dissociation of ligand is rds, will be first order in that species, but zero order in the other. if rds is attack of second species, will be first order in it, zero in the other. If the bridging ligand has more than one atom, can get different coordination of ligand (remote attack). this species could not be synthesized any other way [(NH3)5Co─C≡N:] [Co(CN)5] Co─C≡N─Co [(NH3)5Co] [C≡N─Co(CN)5]-3 Co─SCN: Cr → Co─SCN─Cr N C S Co Cr

8 Kinetics of Octahedral Substitution.
There is not enough data to form a continuous series, but metal ions can be put into four classes based on water-exchange rates. Extremely Fast; k ≈ 108 sec-1 e.g. alkalai metals and larger alkaline earths Fast; k ≈ 105 to 108 sec-1 e.g. dipositive transition metals and tripositive lanthanides Relatively slow; k ≈ 1 to 104 sec-1 e.g. most of the tripositive transition metals, Be+2 and Al+3 Slow (kinetically inert); k ≈ 10-1 to 10-9 sec-1 e.g. Cr+3 (d3); Co+3 (l.s. d6); Pt+2 (l.s. d8); Rh+2; Ru+2 note: these metals are “inert” because they have very high LFSE and either half or filled subshells; any perturbation would cause crystal field to become less stable. Q: Why would tripositive metal ions be more stable than dipositive ones? A: Increased charge on metal strengthens M-L bond.

9 Octahedral Ligand Substitution Reactions
Coordination Sphere Theory It is easier to understand Oh reactions if we look at what is going on around the metal in terms of coordination spheres. 1st sphere: coordinated ligands (L). 2nd sphere: solvent and other molecules (X) held nearby via hydrogen bonds/polar interactions (much weaker than 1st sphere). 3rd sphere: balance of solvent (V). Substitution in Oh complexes is an exchange of species between the 1st and 2nd coordination spheres.

10 Two Substitution Reaction Types
Solvolysis: substitution of a ligand by solvent. Anation: replacement of coordinated solvent. Notice that there is NOT direct substitution of one anion for another. Instead, Oh complex first loses a coordinated anion by solvolysis, then the newly coordinated solvent is relaced by another anion in an anation reaction. L L M L* L L M X + X ↔ L* L L M X L L M L’ + L’ ↔ X +H2O Cl- solvolysis anation [L5Co─Br] [L5Co─OH2] [L5Co─Cl]+2

11 Two Possible Mechanisms
Associative (SN2) [N5ML] + E [N5MLE] [N5ME] + L Dissociative (SN1) [N5ML] [N5M] + E [N5ME] How could one determine experimentally whether the reaction is Associative or Dissociative? Answer: Determine the rate law. slow fast leaving entering coordinate slow fast -L coordinate if Associative: rate = k [N5ML] [E] if Dissociative: rate = k [N5ML] initial concentration of E will not affect the rate.

12 Two Possible Mechanisms
~8 order of magnitude difference in rates. difference in rates less than 2x. These data indicate a dissociative mechanism. In fact, rates correlate well with Co-L bond strengths indicating that this bond is broken initially (i.e. the weaker the bond, the greater the value of k). NOTE: Although you might expect to see a trans-effect in Oh complexes, THERE IS NONE! (i.e. don’t apply square planar rules here)

13 Template Effect method for producing macrocyclic ligands.
however, if throw in a metal. the metal is holding the reactants together in required orientation. the metal holds sulfurs in correct position to allow reaction with both Br’s. Can not do this without metal; get polymer can’t even make starting material without the metal.

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