CHEM 528 (061) Inorganic and Organometallic Reaction

Mechanism Major Exam

NAME ID #

1. Anation of [Cr(NH3)5(H2O)]3+ is thought to proceed by an

increasing mechanism. The table below presents rate constants

and activation parameters for interchange of the entering and

leaving groups in the Cr coordination sphere at 50oC. _______________________________________________________________

Entering ligand 104 k(s-1) H (kJ/mol) S (J/Mol K)

_______________________________________________________________

NCS- 6.12 102 12

HC2O-4 6.2 112 39

C2O2-4 6.2 104 33

H3PO4 1.45

H2PO4- 1.45

[Co(CN)6]3- 2.5 103 26

H2O (exchange) 13.7 97 0

_______________________________________________________________

What do these data suggest about the intimate mechanism of

these reactions?

(ex 11.1)

2. Some volumes of activation for base hydrolysis are given

below. Rationalize the trends observed. ______________________________________________________ Complex V (cm3/mol ______________________________________________________ [Co(NH3)5(O = C(NMe2)H]3+ +43.2 [Co(NH2Me)5Cl]2+ +32.7 [Co(NH2Et)5Cl]2+ +31.1 trans-[Co(en2)Cl2]+ +24.8 cis-[Co(en2)Cl2]+ +27.9 _________________________________________________________

(Ex 1.2)

2. Exchange of NH3 in [Pt(NH3)4]2+ has recently been studied. The reaction

[Pt(NH3)4]2+ + 15NH3 [Pt(15NH3)(NH3)3]2+ + NH3

was followed in buffered aqueous solutions having 0.050 < [NH3]

< 0.50. The rate law was found to be

rate = (k1 + k2[15NH3])[Pt(NH3)4]2+ .

k1 = 3.9 x 10-10 s-1 and k2 = 9.5 x 10-10 M-1 s-1.

(a) What do these data suggest about the exchange mechanism?

(b) Calculate the half-life for ammonia exchange in 1.0 M NH3.

(c) [Au(NH3)4]3+ was found to undergo similar exchange with 15NH3; the rate law is of the same form with k1 = 3 x 10-6 s-1 and k2

= 1.69 x 10-1 M-1 s-1. What can you say about the mechanism of

exchange for AuIII?

(Ex 1.3)

3, Account for the difference in rate constant for the following two reactions:

[Fe(H2O)6]2+ + Cl- [FeCl(H2O)5]+ + H2O k = 106 M-1 s-1

[Ru(H2O)6]2+ + Cl- [RuCl(H2O)5]+ + H2O k = 10-2 M-1 s-1 (p 11.2)

4. The reactions [Cr(NCS)6]3- + solv [Cr(NCS)5(solv)]2- + NCS- have been investigated and found to have the following rate constants near 70oC. _______________________________________________ Solvent k (s-1) ___________________________________________ Dimethylacetamide 9.5 x 10-5 Dimethylformamide 12.4 x 10-5 Dimethylsulfoxide 6.2 x 10-5 _______________________________________________ What do these values suggest about the intimate mechanism of these reactions?

(P 11.7)

5. The following activation parameters have been measured for the reactions [PtClL2X] + py [PtCiL2py]+ + X- in methanol at 30oC. --------- ---------------------------------------------------------------------------------------------------------------- Complex K2 x 103 (M-1 S-1) H (kJ/mol) S (J/mol K) V (cm3/mol) trans-[Pt(NO2)py2Cl] 7.35 49.3 -94 -8.8 cis-[Pt(NO2))py2Cl] 0.150 55.2 -110 -19.8 trans-[Pt(PEt3)2Cl2 0.53 53.9 -100 -13.6

What mechanistic information can be extracted from these values? ( P 11.21)

6. Activation volumes for acid hydrolysis of cis-[PtCl2(NH3)2] and [PtCl2en] are -9.5 (at 45oC) and -9.2 (at 41oC) cm3/mol, respectively. How are these values consistent with what you know about the mechanism for square-planar substitution?

(P 11.22)

7. The following rate constants (in M-1 s-1) were measured for reduction of Co complexes: ---------------------------------------------------------------------------------------------------- Complex [Ru(NH3)6]2+ [Ru(NH3)5(H2O)]2+---------------------------------------------------------------------------------------------------- [Co(N3)(CN)5]3- 1.71 2.06 [Co(NCS)(CN)5]3- 0.81 099 ---------------------------------------------------------------------------------------------------- Comment on the inner or outer-sphere mechanism of these four reactions

(P 11.28)

8. The presence of excess L = PMe2Ph catalyze the isomerization

of cis-[Pt(Cl)2L2] to trans-[Pt(Cl)2L2]. Suggest a mechanism for

this reaction

(P 11.34)