The quadruply bonded metal-metal complexes
cis-Mo
2Cl
2(6-mhp)
2(PR
3)
2(R
3 = Et
3, Me
3,Me
2Ph, MePh
2; 6-mhp= 2-hydroxy-6-methylpyridinato) photoreact when their solutions areirradiated with visible and near-UV light.The primary photoprocess leads to the ligand redistributionproductsMo
2Cl
3(6-mhp)(PR
3)
3and Mo
2Cl(6-mhp)
3(PR
3). In THF at room temperature, these photoproductsare stable and over time they back-react completely tothe starting material. Photolysis of
cis-Mo
2Cl
2(6-mhp)
2(PR
3)
2in DMF results in the same products; however,Mo
2Cl
3(6-mhp)(PR
3)
3rapidly decomposes, leavingMo
2Cl(6-mhp)
3(PR
3)as the only isolable photoproduct.Conversely, when the reaction is carried out in benzene,Mo
2Cl(6-mhp)
3(PR
3)undergoes a slow secondaryphotoreaction andMo
2Cl
3(6-mhp)(PR
3)
3is the photoproduct that is isolated. At a given wavelength, thephotolysisquantum yield (
p) increases along the solvent seriesC
6H
6 < THF < DMF(
p405 = 0.00042, 0.00064, and0.00097, respectively, for
cis-Mo
2Cl
2(6-mhp)
2(PMe
2Ph)
2).For a given solvent,
p increases withdecreasingexcitation wavelength (
p546 = 0.00012,
p436 = 0.00035,
p405 = 0.00042,
p366 = 0.0022, and
p313 = 0.0079in C
6H
6). This wavelength dependence ofthe photoreaction quantum yield in conjunction with theexcitationspectrum establishes that the photoreaction does not originate from thelowest energy
* excited state, whichpossesses a long lifetime and an appreciable emission quantum yield inC
6H
6, CH
2Cl
2, THF,and DMF. Thephotochemistry is instead derived from higher energy excited stateswith the maximum photoreactivity observedfor excitation wavelengths coinciding with absorption featurespreviously assigned to ligand-to-metal charge transfertransitions.
