Rhenocene, Re(?75-C5H5)2, has been generated by photolysis of Ref^-CsHs^H isolated in nitrogen and argon matrices at 12 K, and has been studied by laser-induced fluorescence with a pulsed tunable laser. The emission lifetime of the lowest LMCT excited state is (71.9 ± 1.3) ns in nitrogen and (69.3 ± 1.4) ns in argon matrices.The emission and excitation spectra are far simpler and much better resolved than the UV/vis absorption spectrum, because the laser selects individual sites/conformers in the matrix (or groups of sites with very similar absorptions). Vibrational progressions are detected which are based on the ring-metal-ring stretching mode, v4, and combinations of that mode with the symmetric ring breathing mode, v2, and the symmetric C-H deformation mode, v3. In the LMCT excited state, there is an increase in v4 of 5% and a decrease in v3 (9%) and v2 (3%), consistent with transfer of an electron from a ligand jr-orbital to a metal-ligand bonding orbital.

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The reactive metallocenes, tungstenocene and molybdenocene (M($-CsH5)2; M = Mo, W), have been generated by photolysis of the corresponding dihydnde complexes, M($-C~H~)ZHZ, in argon and nitrogen matrices at 12 K. The metallocenes have been probed by laser-induced fluorescence with a pulsed tunable laser and by UV/vis absorption spectroscopy. Structured emission is observed from the LMCT excited states (lifetimes 10 ns). The spectra are complicated by multiple sites/conformers, but emission spectra of a single site/ conformer may be obtained with appropriate selection of matrix and excitation wavelength. Corresponding excitation spectra are measured from the area of selected emission peaks as a function of excitation wavelength.Vibrational progressions are dominated by the ring-metal-ring symmetric stretching mode (v4 x 300 cm-l).Nevertheless, this mode changes in frequency by 1 4 cm-' (51.3%) in the LMCT excited state. The bestresolved peaks have a full width at half-maximum of ca. 10 cm-'. Most of the emission is vibrationally fully relaxed, but weak emission peaks arising from u' = 1 states are found for MoCpz in N2 matrices.

Resonance Raman spectroscopy has been used to probe the structures of; tetrakis(1-methylpyridinium-4-yl)porphi natoi ron( II I), Fel I '(T4MPyP) ; tetrakis( 1 -methyl pyridium-2-y1)porphi natoi ron(iii), Fell '(T2MPyP) ; tetrakis(4sulphonatophenyl)porphinatoiron(iii), Fe'"(TSPP); and tetrakis(4-carboxylatophenyl)porphinatoiron(iii), Fe"'(TCPP), over a wide pH range. The anionic complexes Fe"'(TSPP) and Fe"'(TCPP) contain high-spin iron(iii) at all pHs. Both these complexes exhibit marked spectral changes at ca. pH 6 which correspond to conversion from the diaquo species, in acid solution, to hydroxy-or p o x 0 dimer complexes. Both cationic complexes show similar diaquo to high-spin hydroxy, or p-0x0 dimer, transitions at ca. pH 6. However, at pH > 11.5 for Fe'"(T4MPyP) and pH > 9 for Fel"(T2MPyP) a second equilibrium process is observed, leading to two new species. One of these is readily assigned as the low-spin iron(iii) dihydroxy complex by analogy with spectra of the dicyano complex. The second species is assigned to the hydroxy iron(ii) complex by comparison with photochemically generated FeI1(T4MPyP)(OH). The formation of iron(\\) species in alkaline solutions of Fe'"(T4MPyP) and Fe"'(T2MPyP) is entirely unexpected and the significance of the observation to previous investigations of the pH-dependent behaviour of these complexes is discussed.

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