Photoinduced anti—syn isomerization in tungsten carbonyl carbenes, (CO)5W=C(OR)R', [1 with R = Me, R' = Me; 2 with R = Me, R' = p-Tolyl; 3 with R = Et, R' = Ph] brought about by irradiation in the MLCT absorption region has been investigated by laser flash photolysis coupled with transient absorbance and time-resolved resonance Raman spectroscopy (TR3S). The studies, carried out in solution in solvents of varying polarity, have been supplemented by matrix isolation studies using UV-vis and IR detection. Pulsed irradiation in the MLCT absorption region of 1-3 results in the formation within the pulse risetime of the syn isomer from the ground state (anti) form, followed by thermal relaxation/isomerization back to the ground state. In all three complexes the transient lifetimes increase with increasing solvent polarity falling in the range 120 μs-3 ms for 1 and 1–2 orders of magnitude shorter for 2 and 3. Growth and decay of the syn isomer were also monitored by TR3S. For 1, bands in the Raman attributable to modes of the carbene group showed little change in frequency between the anti and syn isomers. In the case of the anti forms of 2 and 3, a band near 1235 cm-1 attributable to ν(CCarbene-OR) shifts to ca. 1270 cm-1in the spectra of the syn isomer. The differences beween 1,2 and 3 in respect of both vibrational spectra and rates of isomerization are discussed in relation to the possible influence on electronic stabilization of reorientation of the R' group which accompanies anti-syn transformation. Comparison of the anti—syn photoisomerization of 1 in an argon matrix with that of 4 (R = Me, R' = Ph) shows that the process is photoreversible only in the case of 1. © 1995, American Chemical Society. All rights reserved.