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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.

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.

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The effects of hydrostatic pressure on the luminescence spectra of tetragonal transition metal complexes with nondegenerate electronic ground states are analyzed quantitatively using models based on potential energy surfaces defined along normal coordinates. Pressure-induced changes of intensity distributions within vibronic progressions, band maxima, electronic origins and relaxation rates are discussed for metal-oxo complexes of rhenium(V) and molybdenum(IV) (d 2 electron configuration) and for square-planar complexes of palladium(II) and platinum(II) (d 8 electron configuration).