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Gas-Phase Ligand Exchange in a Square-Planar Rhodium(I) Complex Proceeding by Dissociative Exchange:  ESI FT-ICR MS and DFT Evidence

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journal contribution
posted on 05.01.2004, 00:00 by Robert E. Bossio, Norris W. Hoffman, Thomas R. Cundari, Alan G. Marshall
Although the kinetics of square-planar d8 ligand-substitution reactions have been explored extensively in solution, no such studies of gas-phase systems have been reported. Measurement of gas-phase ligand exchange offers the advantage that the exchange mechanism is readily determined in the absence of solvent effects. The complex trans-Rh(PPh3)2CO(4-picoline)+ substitutes pyridine for its 4-picoline ligand readily in the gas phase but also loses CO during the reaction. The double-resonance experiments show that its primary substitution pathway proceeds dissociatively, unlike almost all solution ligand substitutions for Pt(II) and Pd(II). The density functional calculations reveal that the bond energies of Rh-4-picoline, Rh-pyridine, and Rh−CO are similar enough that CO loss might be expected. In support of the dissociative mechanism, the calculations also show five-coordinate intermediates for the associative pathway to be relatively high in energy.

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