Organometallic Solutions: #8

8.* (1994 F 6) Anhydrous chromium trichloride (CrCl3) is insoluble; in the solid state CrCl3 exists as a three-dimentsional polymeric bridging chloride structure. By contrast, the hydrate, CrCl3(H2O)3 is soluble in water; it does not exchange with radioactive chloride anions over a period of many days in solution. Chromium dichloride (CrCl2) is also soluble in H2O; the chloride ligands in CrCl2 are rapidly exchanged by H2O forming Cr(H2O) x2+.

A. Propose a mechanism for this process by writing equations for the steps which may occur. (Hint: Eo for Zn2+/Zn- is -0.7 V vs. NHE.)

Zn metal is used to produce a trace amount of Cr2+ in solution which then catalytically solubilizes CrCl3 through electron transfer (Cr(III) is inert to substitution, Cr(II) is labile).

B. Give a rationale for the difference in the rates of chloride exchange comparing Cr(III) and Cr(II).

Cr(III) in Oh field --> d3

Cr(II) in Oh field --> d4

Cr(III) has 1/2 filled shell stability => inert

Cr(II) does not => far more labile!

i.e. faster ligand substitution

C. Discuss the number and nature of isomers formed by CrCl3(L)3. Draw structural formulas depicting these stereoisomers.

fac

mer

 

2 stereoisomers

(diastereomers)

D. How would you separate such stereoisomers?

Since they are diastereomers, they can be separated via standard techniques (physical / chemical differences): crystallization, chromatography…