page_title: Rhodocene
section_title: The [(η5-C5tBu3H2)Rh(η5-C5H5)]+ cation
hierarchical_section_title: Rhodocene / Substituted rhodocenes and rhodocenium salts / The [(η5-C5tBu3H2)Rh(η5-C5H5)]+ cation
caption_attribution_description: English: This figure is based on data presented in the following paper and also provided in its supplementary materials document: doi:10.1021/om9707735. Part of the diagram is adapted from a Figure 4 in that paper, the rest is my own work. It was prepared using Microsoft PowerPoint 2003. Given that the data themselves are not copyright, I release the diagram into the public domain.
context_page_description: Rhodocene, formally known as bisrhodium(II), is a chemical compound with the formula [Rh(C₅H₅)₂]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has covalent rhodium–carbon bonds. The [Rh(C₅H₅)₂] radical is found above 150 °C or when trapped by cooling to liquid nitrogen temperatures. At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.
  The history of organometallic chemistry includes the 19th-century discoveries of Zeise's salt and nickel tetracarbonyl. These compounds posed a challenge to chemists as the compounds did not fit with existing chemical bonding models. A further challenge arose with the discovery of ferrocene, the iron analogue of rhodocene and the first of the class of compounds now known as metallocenes. Ferrocene was found to be unusually chemically stable, as were analogous chemical structures including rhodocenium, the unipositive cation of rhodocene and its cobalt and iridium counterparts.
context_section_description: Novel approaches to synthesising substituted cyclopentadienyl complexes have been developed using substituted vinylcyclopropene starting materials. Ring-enlarging vinylcyclopropane rearrangement reactions to produce cyclopentenes are well known and serve as precedent for vinylcyclopropenes rearranging to cyclopentadienes. The [(η⁵-C₅ᵗBu₃H₂)Rh(η⁵-C₅H₅)]⁺ cation has been generated by a reaction sequence beginning with addition of the chlorobisethylenerhodium(I) dimer, [(η²-C₂H₄)₂Rh(μ-Cl)]₂, to 1,2,3-tri-tert-butyl-3-vinyl-1-cyclopropene followed by reaction with thallium cyclopentadienide:
  The 18-valence electron rhodium(III) pentadienediyl species generated by this reaction demonstrates again the instability of the rhodocene moiety, in that it can be refluxed in toluene for months without 1,2,3-tri-tert-butylrhodocene forming but in oxidising conditions the 1,2,3-tri-tert-butylrhodocenium cation forms rapidly. Cyclic voltammetry has been used to investigate this and similar processes in detail. The mechanism of the reaction has been shown to involve a loss of one electron from the pentadienediyl ligand followed by a fast rearrangement (with loss of a hydrogen atom) to form the 1,2,3-tri-tert-butylrhodocenium cation. Both the tetrafluoroborate and hexafluorophosphate salts of this cation have been structurally characterised by X-ray crystallography.
  [(η⁵-C₅ᵗBu₃H₂)Rh(η⁵-C₅H₅)]BF₄ forms a colourless centrosymmetric monoclinic crystal belonging to the P2₁/c space group, and with a density of 1.486 g cm⁻³. Looking at the ORTEP diagram of the structure of the cation (at right), it is evident that it possesses the typical geometry expected of a rhodocene or rhodocenium cation. The two cyclopentadienyl rings are close to parallel (the centroid–Rh–centroid angle is 177.2°) and the rhodium centre is slightly closer to the substituted cyclopentadienyl ring (Rh–centroid distances are 1.819 Å and 1.795 Å), an observation attributed to the greater inductive effect of the tert-butyl groups on the substituted ligand. The ORTEP diagram shows that the cation adopts an eclipsed conformation in the solid state. The crystal structure of the hexafluorophosphate salt shows three crystallographically independent cations, one eclipsed, one staggered, and one which is rotationally disordered. This suggests that the conformation adopted is dependent on the anion present and also that the energy barrier to rotation is low – in ferrocene, the rotational energy barrier is known to be ~5 kJ mol⁻¹ in both solution and gas phase.
  The diagram above shows the rhodium–carbon (in red, inside pentagons on the left) and carbon–carbon (in blue, outside pentagons on the left) bond distances for both ligands, along with the bond angles (in green, inside pentagons on the right) within each cyclopentadienyl ring. The atom labels used are the same as those shown in the crystal structure above. Within the unsubstituted cyclopentadienyl ligand, the carbon–carbon bond lengths vary between 1.35 Å and 1.40 Å and the internal bond angles vary between 107° and 109°. For comparison, the internal angle at each vertex of a regular pentagon is 108°. The rhodium–carbon bond lengths vary between 2.16 Å and 2.18 Å. These results are consistent with η⁵-coordination of the ligand to the metal centre. In the case of the substituted cyclopentadienyl ligand, there is somewhat greater variation: carbon–carbon bond lengths vary between 1.39 Å and 1.48 Å, the internal bond angles vary between 106° and 111°, and the rhodium–carbon bond lengths vary between 2.14 Å and 2.20 Å. The greater variation in the substituted ligand is attributed to the distortions necessary to relieve the steric strain imposed by neighbouring tert-butyl substituents; despite these variations, the data demonstrate that the substituted cyclopentadienyl is also η⁵-coordinated.
  The stability of metallocenes changes with ring substitution. Comparing the reduction potentials of the cobaltocenium and decamethylcobaltocenium cations shows that the decamethyl species is ca. 600 mV more reducing than its parent metallocene, 
caption: Selected bond lengths (Å) (left) and bond angles (°) (right) for the unsubstituted (top) and substituted (bottom) cyclopentadienyl ligands in the cation from the salt [(η5-C5tBu3H2)Rh(η5-C5H5)]BF4.[45]
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