Problem Set #6

Level 1 Problems:
1. Write the ligand substitution mechanism for the following reaction :

[Co(NH3)5(Br)]2+ + CN- --> [Co(NH3)5(CN)](2)+ + Br-

[Co(NH3)5(Br)]2+ --> [Co(NH3)5]3+ + Br- because Co(3+) substitutions always dissociative

[Co(NH3)5]3+ + CN- --> [Co(NH3)5(CN)]2+ AND

[Co(NH3)5]3+ + H2O- --> [Co(NH3)5(H2O)]3+ because rate of water exchange is faster than CN- and

because there's a high concentration of water to compete with CN- addition. Then:

[Co(NH3)5(H2O)]3+ --> [Co(NH3)5]3+ + H2O so the five coordinate intermediate can be generated from the aquo complex and is free to be attacked by CN-.

2. Suggest an explanation for the following observations:

The rate of substitution of the aquo ligand of [Co(NH3)5(H2O)]3+ by either Cl- or NCS- is the same.
The rate of aquo substitution of [Rh (NH3)5(H2O)]3+ by Br- is 5 times faster than aquo substitution by SO42- .

Co(3+) substitutions are always dissociative and the observation that the rate is uncahnged by the incoming ligands is consistant with that.

Since the rate of Rh(3+) subsitution depends on the incomig ligand, it must be true that the mechanism is associate instead.

3. Rank the following hexaaquo -metal ions in order of increasing inertness, and give a few word summary to account for the position of each ion in the sequence.

Co(2+), Co(3+), Pt(2+), Fe(3+), Cr(2+), V(2+), Ir(3+)

Cr(2+) > Co(2+) > Fe(3+) > V(2+) > Co(3+) > Pt(2+) > Ir(3+)

Cr2+ is most labile due to Jahn-Teller distortion and weak axial bonds

Co2+ is lablile because low charge and no special CF stability

Fe3+ is less labile due to higher charge

V2+ is d3, which makes it less labile than other 2+ ions

Co(3) is very stable as a d6 metal with 3+ charge.

Pt2+ is 3rd row so inert

Ir3+ is 3rd row so inert, it has stable low spin d6 CFSE and has higher charge than Pt2+

3. Use the Latimer diagram to determine:

• the most stable oxidation state of vanadium
• if any oxidation state of vanadium is unstable with respect to disproprotionation

VO2+  ----------->  VO2+   ----------->  V3+  ------------->  V2+ ----------->  V
         +1.0V                       +0.34V             -0.26V             -1.13V

V does not disproportionate at any oxidation state because the sum of the oxidation + reduction potentials give (-) values. The most stable oxidation state is V3+ since there are negative potentials to both oxidize and reduce it.

Level 2 Problems:
4. (a) Draw the structure of the delta enantiomer [Co(en)2(Cl)2]+
(b) An aqueous solution of delta [Co(en)2(Cl)2]+ that has been at room temperature for 24 hours no longer rotates polarized light and mass spectrometry analysis shows it has undergone ligand substitution to form [Co(en)2(Cl)(H2O)]2+ .  Provide a mechanism consistent with these observations.

The loss of rotation of polarized light means that racemization has occurred, as well as formation of achiral trans-[Co(en)2(Cl)(H2O)]2+. Racemization indicates that a stabilized CN=5 intermediate of trigonal bipyramidal geometry has formed. Attack on this trig.bpy intermediate can occur at different sites to produce both thedelta and lambda enantiomers and the trans isomer. click for diagram

5. For the reaction:
[Co(NH3)5(X)]2+ + [Cr(H2O)6]2+ +  5[H3O]+ --->  [Co(NH3)5(X)]2+ +  [Cr(H2O)5(X)]2+ +  5 [H4N]+   

 the rate constant for X = Cl- , k = 6.0 x 10^5 while for X = I- , k = 3.0 x 10^6 M-1s-1.

Suggest a mechanism for the reactions and speculate on what is the rate determining step.  Include a comment on why the rate for chloride and iodide differ.

These are relatively fast electron transfer (redox) reactions. That information and the fact that Cl- and I- are both potentially bridging ligands suggests an inner sphere reaction. The observation that the iodide reaction is slightly faster can be rationalized as it making a weaker bond to the metals making the final dissociation step faster. Note that one reactant is labile (Cr) providing a way to open a coordination site to make the X-bridge.

[Cr(H2O)6]2+ --->   [Cr(H2O)5]2+ + H2O dissociate of labile complex

[Co3+(NH3)5(X)]2+ + [Cr2+(H2O)5]2+--->  [(NH3)5Co3+—(X)]—[Cr2+(H2O)5]4+

[(NH3)5Co3+—(X)]—[Cr2+(H2O)5]4+ ---> [(NH3)5Co2+—(X)]—[Cr3+(H2O)5]4+ electron transfer step

[(NH3)5Co2+—(X)]—[Cr3+(H2O)5]4+ --->  [Co(NH3)5]2+ +  [Cr3+(H2O)5(X)]2+ dissociation of bridged species at more labile metal, Co(2+)

[Co(NH3)5]2+ +  5[H3O]+ ---> [Co(H2O)6]2+ +  5 [H4N]+   ligand substitution of NH3 by H2O