Question

P(Me)3 P(Me)3 Re 0 3. (14 pts) For the rhenium complex A, a change in hapticity of the cyclopentadienyl (Cp) ligand occurs with the addition of P(Me)3. Answer the following questions about this phenomenon. How many resonances do you expect to see in the 1H NMR spectrum of the η 1 a. complex C? Explain. b. The researchers who observed this phenomenon confirmed the structure of complex C by X-ray diffraction analysis. At room temperature, the H NMR spectrum of the compound only displayed one signal for the Cp ligand (at 6.15 ppm). Is that consistent with ηι coordination of the Cp ligand? Provide a brief explanation for this observation. c. The researchers also performed the H NMR experiments of C at low temperature (-87 °C), and still observed the same solitary signal as in the room-temperature spectrum. Briefly explain this observation.

Answer 1

(a) The complex C has octahedral geometry, with two CO ligands trans to each other, two P(Me₃)₃  ligands trans to each other and the third CO occupying the opposite side of the η¹-Cp ligand in the axial position. So all the four protons of the η₁-Cp ligand are magnetically equivalent. Therefore we expect a single resonance in the ¹H NMR spectrum of the η¹- complex C.

(b) Yes, one signal for the Cp ligand in the ¹H NMR spectrum of the η¹- complex C is consistent with the η¹- coordination of the Cp ligand, because due to the η¹- coordination of the Cp ligand, all the four protons of Cp experience the same magnetic environment in the octahedral geometry of the complex C. So, only one signal is observed in the ¹H NMR spectrum of the η¹- complex C.

Moreover, at room temperature, the ligands at the equatorial positions keep interchanging their cis-trans positions at a very high frequency. So, all the four protons of the η¹-Cp ligand are under the influence of same magnetic environment and therefore only one signal is observed in the ¹H NMR spectrum of the η¹- complex C.

(c) As discussed in (a) and (b), the η¹-complex C has octahedral geometry, with two CO ligands trans to each other, two P(Me₃)₃  ligands trans to each other and the third CO occupying the opposite side of the η¹-Cp ligand in the axial position. So all the four protons of the η₁-Cp ligand are magnetically equivalent even at low temperature. Therefore, the same solitary signal is observed in the ¹H NMR spectrum of the η¹- complex C as in the room temperature spectrum.