Homework 2, due October 9, before class. 24 points total
1) Upon dissolution of A in CD3CN, a sluggish (k = 1.2 x 10-8 s-1) exchange with CH3CN occurs to give complex B. The reaction rate was found to be independent of [CD3CN]. The activation parameters were ΔH‡ = 37.2 kcal/mol and ΔS‡ = +30.2 e.u..
Based on the given data, does this appear to be an associative or dissociative ligand substitution reaction? Explain. (4 points)
The reactivity of a similar iridium complex was investigated. It was found that the reaction with a Tp ligand where R = Me occurred with a very high rate (t1/2 < 5 s at 0 °C), while the unsubstituted Tp complex had a much slower ligand substitution rate (t1/2 = 75 min at room temp). Based on this result, does this complex undergo ligand substitution by the same mechanism as the ruthenium complex, or by another mechanism. Explain. (4 points)
2) Treatment of (C2 H4)Ni(P(OAr)3)2 with excess C2 H4 and hydrogen cyanide leads to the formation of an equilibrium mixture of the complexes A, B, and C (of random ratio 0.14:0.8:0.06), based on the appearance of three resonances in the 31 P NMR spectrum, with C as the major isomer.
When the equilibrium mixture of complexes A-C was treated with tri(o-tolyl)phosphite, reductive elimination occurred to regenerate the original ethylene complex and produce propionitrile. Study of this reductive elimination showed that: ΔH‡ = 8.9 kcal/mol and the ΔS‡ = -34 eu (at –40 °C). The reaction was found to have a first order dependence on tri(o-tolyl)phosphite concentration.
a) Provide a detailed mechanism for this reductive elimination using the major isomer as your example. Briefly explain how the data supports your mechanism. (4 points)
b) Can reductive elimination occur effectively from each of the A-C structures? Explain why or why not. (2 points)
3) Which of the following statements concerning the below reaction is not true? (4 points)
a) The reaction proceeds through a five coordinate intermediate.
b) In the final product, the trans influence of the methyl group makes the M-CO bond weaker.
c) The rate of the reaction shows a dependence on [Ni].
d) The rate of the reaction shows a dependence on [CO].
e) The reaction would be slower if the methyls were replaced with mesityls:
4) In the substitution of V(CO)6, the rate of reaction changes with respect to phosphine nucleophile according to the order PMe3 > PBu3 > P(OMe)3 > PPh3. What does this suggest about the mechanism? (2 points).
5) Which of the following two complexes is predicted to be more reactive towards ligand substitution by an associate pathway and why? (4 points)