CHEM 263 Recitation
Week 6: Mechanisms in Coordination Chemistry
Name: ________________________________
WS-6
The trans effect problem set was adapated from one created in December 2014 by Sheri Lense (University of Wisconsin Oshkosh,
[email protected]). Copyright 2015. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike License.
To view a copy of this license visit http://creativecommons.org/about/license/.
The trans-directing series:
The Importance of the Trans Effect in the Synthesis of Novel Anti-Cancer Complexes
Three square-planar Pt(II) complexes are currently approved by the F.D.A. for use in cancer treatment, the most
famous of which is cisplatin, or cis-diamminedichloroplatinum(II). While cisplatin can be used to treat several
different types of prostate cancers, one issue is that many patients can relapse with a cisplatin-resistant disease.
Therefore, it is important to develop new chemotherapeutic drugs that demonstrate efficacy in cisplatin-resistant
cell lines. Square planar Pt(II) complexes that contain mixed cis-amine/ammine or cis-pyridyl/ammine motifs are
of interest as both potential novel chemotherapeutic Pt(II) complexes
and as intermediates for promising chemotherapeutic drugs such as
satraplatin. An example of a Pt(II) complex with a cis-amine/ammine
motif is shown in Figure 1. The trans effect can be very useful in the
development of methodologies for the selective synthesis of the Figure 1. cis-[Pt(NH3)(NH2C6H11)Cl2]
desired cis isomer.
One way to synthesize cis-ammine/amine Pt(II) complexes is by beginning with [Pt(NH3)Cl3]−. The synthesis of
cis-[Pt(NH3)(NH2C6H11)Cl2] was described by Giandomenico et al. in 1995.1 In this synthesis, shown below in
Scheme 1, the mixed-halo complex 2 is first isolated, and then converted to the dichloride complex 3 by
formation of the aqua species with silver nitrate, followed by precipitation with HCl.
1.
Christen M. Giandomenico et al. Inorganic Chemistry 1995, 34, 1015-1021.
Scheme 1. Synthesis of cis-[Pt(NH3)(NH2C6H11)Cl2].1
1) The trans effect in synthesis
a. Would this isomer of complex 2 be predicted to form based on the trans effect? Show your work.
b. Why is the geometry of complex 2 necessary for formation of cis-[Pt(NH3)(NH2C6H11)Cl2]?
Classifying reaction mechanisms
3) Consider the following two compounds, [Cr(H2O)6}3+ and [Cu(H2O)6]2+. Both will go through a ligand
exchange reaction with isotopically labeled water and will go through a five-coordinate intermediate (square
pyramidal geometry) as their rate-limiting step.
a. What mechanism do the ligand exchanges follow? Briefly explain.
b. Draw d-orbital splitting diagrams for the intermediates, one for each complex.
c. One of the complexes has a faster rate of a ligand exchange than the other. Without actually doing the
calculation, explain how you would determine which metal likely undergoes a faster ligand exchange.
The complexes [Fe(CN)6]4- and [IrCl6]2- typically react slowly with other compounds. However, the two
complexes react with each other very quickly.
d. What kind of a reaction is this, and what mechanism does it follow? Briefly explain.
e. Why might this reaction be so fast, while many reactions with these complexes are slow?
4) The following reaction occurs, where X- is a halide ligand.
[CoX(H2O)5]2+ + [Cr(H2O)6]2+ → [Co(H2O)6]2+
+ [CrX(H2O)5]2+
It can be experimentally shown (by isotopically labeling X) that the X- bound to the Co3+ in [CoX(H2O)5]2+ is the
same X- that is bound to Cr3+ in [CrX(H2O)5]2+.
a. What changes in this reaction?
b. What kind of a reaction is this, and what mechanism does it follow? Briefly explain.
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