Reactions of niobium and tantalum monoxide and dioxide molecules with monochloromethane in solid argon have been investigated by infrared absorption spectroscopy and density functional theoretical calculations. The results show that the ground-state MOx (M = Nb, Ta, x = 1, 2) molecules react with CH3Cl to form the weakly bound MO(CH3Cl) and MO2(CH3Cl) complexes. The MO(CH3Cl) complexes rearrange to the more stable CH2ClM(O)H isomer upon visible light excitation, whereas the MO2(CH3Cl) complexes isomerize to the more stable CH2ClM(O)OH molecules under ultraviolet light irradiation. The CH2ClM(O)H and CH2ClM(O)OH molecules were predicted to involve agostic interactions between the chlorine atom and the metal center.