The Structure and Symmetry of Metal Tris Chelates


Using the Jmol (a free open source) Program to Display 3D Images and Present Interactive Content
Authors: Marion E. Cass, Carleton College and Henry S. Rzepa, Imperial College London
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Introduction: Achieving understanding of complicated molecular phenomena from two dimensional drawings can be a challenge. Three examples that we have struggled with, and would like to highlight in our animated visualizations, are the C2 and C3 symmetry operations in chiral tris chelates, the assignment of absolute configurations in these molecules and the non-dissociative mechanisms (the Bailar Twist and the Ray-Dutt) that interchange Λ and Δ enantiomers. Using three dimensional representations of the molecules that can be exported from the Cambridge Structural Data Base, computational output examining the imaginary frequencies of transition states, and the powerful molecular visualization program Jmol, we present here a web site that we have constructed to aid teaching and learning these concepts. In addition, we have included animations that show the parallels between left and right handed propellers, a discussion of the structural aspects of δ and λ twists in ligand rings (possible in some chelates), and a series of exercises to help students develop their skills in understanding the structural features of these metal tris chelates. Two examples of published research are included to illustrate the stereoselective interactions of synthetically designed and/or naturally occurring chiral metal complexes with relevant biological molecules. References and simple descriptions of the metal ligand bonding are given for the exercises and examples in linked pages.
A. What is a Metal Tris Chelate E. Analogies to Propellers
B. Symmetry Operations for Metal Tris Chelates
  1. With Symmetric Ligands
  2. With Asymmetric Ligands
  3. Higher Order Symmetries
 F. Metal Tris Chelates with Non-Planar Ligands that can have λ or δ Ligand Conformations
  1. What is a λ or a δ Ligand Conformation?
  2. Contrasting the Λ -λλλ and Δ -δδδ Enantiomers of Co(H2NCH2CH2NH2)33+
  3. The Eight Isomeric Forms of [Ni(H2NCH2CH2NH2)3]2+
C. Stereochemical Properties of Metal Tris Chelates
  1. Definition of Chirality
  2. Δ and Λ Enantiomeric Forms
  3. Assigning the Absolute Configuration (Δ or Λ)
      a. Method 1
      b. Method 2
G. Mechanisms that Racemize the Λ and Δ Forms of Metal Tris Chelates
  1. Non-dissociative Mechanisms
  2. Knowing that Dissociative Mechanisms are Very Common
      Ligand substitution is a very common mechanism that results in the racemization of the enantiomers in kinetically labile metal complexes.
      A discussion of ligand substitution reactions and kinetically labile species can be found in most inorgnaic texts. See
      1. Miessler, G.L., Tarr, D.A.; Inorganic Chemistry, 3rd Edition, Pearson Prentice Hall, Upper Saddle River, NJ, 2004, Chapter 12.
      2. Atkins, P., Overton, T., Rourke, J, Weller, M, Amstrong, F, Salvador, P., Hagerman, M., Sprio, T., Stiefel, E.; Inorganic Chemistry, 4th Ed, W.H. Freeman and Company, New York, NY, 2006, Chapter 20.
      3. Housecroft, C. E., Sharpe, A.G.; Inorganic Chemistry, 2nd Edition, Pearson Prentice Hall, Upper Saddle River, NJ, 2005, Chapter 25.
D. Examples of Research Applications
  1. Chiral Interactions with DNA
  2. Chiral Interactions with Proteins
 H. Exercises
  1. One Isomer of Ir(acac)3
  2. One Isomer of Mn(acac)3
  3. One Isomer of Mo(acac)3
  4. One Isomer of [Fe(2-ethylaminopyridyl)3]3+
  5. One isomer of a [Fe(TRENCAM)]3-
  6. A Capped Tris Chelate of Cu2+
  7. Two Isomers with λ / δ ligand twists
  8. One Isomer of [Co(1,2-diaminocyclohexane)3]3+