The Pd(II) dimers [(2-phenylpyridine)Pd(mu-X)](2) and [(2-p-tolylpyridine)Pd(mu-X)](2) (X = OAc or TFA) do not exhibit the expected planar geometry (of approximate D(2h) symmetry) but instead resemble an open "clamshell" in which the acetate ligands are perpendicular to the plane containing the Pd atoms and 2-arylpyridine ligands, with the Pd atoms brought quite close to one another (approximate distance 2.85 A). The molecules adopt this unusual geometry in part because of a d(8)-d(8) bonding interaction between the two Pd centers. The Pd-Pd dimers exhibit two successive one-electron oxidations: Pd(II)-Pd(II) to Pd(II)-Pd(III) to Pd(III)-Pd(III). Photophysical measurements reveal clear differences in the UV-visible and low-temperature fluorescence spectra between the clamshell dimers and related planar dimeric [(2-phenylpyridine)Pd(mu-Cl)](2) and monomeric [(2-phenylpyridine)Pd(en)][Cl] (en = ethylenediamine) complexes that do not have any close Pd-Pd contacts. Density functional theory and atoms in molecules analyses confirm the presence of a Pd-Pd bonding interaction in [(2-phenylpyridine)Pd(mu-X)](2) and show that the highest occupied molecular orbital is a d(z(2)) sigma* Pd-Pd antibonding orbital, while the lowest unoccupied molecular orbital and close-lying empty orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other Pd(II)-Pd(II) dimers that have short Pd-Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(mu-X)](2), but quite different from that found for d(8)-d(8) dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd and may explain why Pd d(8)-d(8) dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species.
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