Reactions and photochemistry of chromium and molybdenum with molecular hydrogen at 12 K

Cr and Mo have been cocondensed with molecular hydrogen in Kr and Ar matrices at 12 K. Both chromium and molybdenum atoms were found to insert into the H-H bond upon absorption of UV light (320-380 nm) to form MH2 (M = Mo, Cr). The symmetric and antisymmetric stretching frequencies have been identified for both CrH2 and MoH2 and the bond angles are estimated to be 118 +/- 5-degrees and 110 +/- 5-degrees for CrH2 and MoH2, respectively, from the relative intensities of the antisymmetric and symmetric stretching modes. The stretching force constants of CrH2 and MoH2 are determined to be 1.64 and 1.86 mdyn/angstrom. A geometry of slightly unequivalent M-H bonds for MHD (M = Cr and Mo) is suggested. A molecular hydrogen adduct, CrH2(H2); is found in reactions with excess hydrogen. Photolysis of the matrices which contain appreciable CrH2(H2) with light of 520 nm < lambda < 580 nm leads to the formation of CrH3. In the molybdenum reactions, MoH, MoH3, and possibly MoH4 were identified as products of UV photolysis as well.