Interaction of thioether groups at the open coordination sites of palladium(II) and platinum(II) complexes probed by luminescence spectroscopy at variable pressure.

The crystal structures of [PtCl(2)(ttcn)], [PdCl(2)(ttcn)], [Pt(ethylenediamine)(ttcn)](PF(6))(2), and [Pd(ethylenediamine)(ttcn)](PF(6))(2) (ttcn = 1,4,7-trithiacyclononane) show short apical metal---S(ttcn) distances, qualitatively indicating an interaction. Luminescence spectroscopy was used to study these crystalline complexes at room temperature and variable hydrostatic pressure. The luminescence band maximum of [PdCl(2)(ttcn)] shows a pressure-induced blue shift of +6 cm(-1)/kbar, while the platinum(II) compounds show a red shift of approximately -20 cm(-1)/kbar. This difference is rationalized in terms of a competition between blue shifts due to pressure-induced metal-ligand bond shortening in the equatorial plane and increasing out-of-plane distance of the metal center, and a red shift due to the approach of the apical sulfur donor to the metal center. Density functional theory (DFT) calculations indicate d-d luminescence transitions and a different nature of the highest occupied molecular orbital (HOMO) for [PdCl(2)(ttcn)] than for [PtCl(2)(ttcn)], while the lowest unoccupied molecular orbitals (LUMOs) are identical in character. This electronic structure difference is used to rationalize the different pressure effects.

Pressure-dependent luminescence spectroscopy of molybdenum(IV) oxo complexes.

Luminescence spectra are reported for trans-[MoOCl(CN-t-Bu)(4)]BPh(4) and trans-[MoOF(pyridine)(4)]BPh(4). The maxima of the broad luminescence bands are at energies of 12000 and 13000 cm(-1), respectively. Resolved vibronic structure involves the metal-oxo stretching mode with a vibrational frequency of approximately 950 cm(-1) (Raman) in both compounds. The pressure behavior of the luminescence bands shows a distinct difference: a blue shift of +12 cm(-1) kbar(-1) is observed for [MoOCl(CN-t-Bu)(4)]BPh(4), and a red-shift of -8 cm(-1) kbar(-1) is obtained for [MoOF(pyridine)(4)]BPh(4). At pressures above 25 kbar, the luminescence maximum of [MoOCl(CN-t-Bu)(4)]BPh(4) shows a change from a blue-shift to a red-shift and the Raman peak corresponding to the metal-oxo stretching mode at 954 cm(-1) becomes broad and its maximum decreases with increasing pressure, leading to a strong deviation from the linear increase of +0.24 cm(-1) kbar(-1) observed at pressures below 25 kbar. These unusual pressure effects are rationalized in terms of the influence of pressure on the metal-ligand bonds and oxo-metal-ligand angles using DFT calculations.