Reactions and photochemistry of transition metals with hydrogen and methane via matrix isolation FT-IR spectroscopy

The reactions of transition and main group metals, including Ca, Sc, Ti, V, Cr, Mo, Zn, Ga and Ge, with molecular hydrogen have been investigated using FT-IR matrix isolation spectroscopy. The reactions of gallium with methane in Ar, Kr and pure methane matrices have also been studied. All the above metals (except Ge) encounter an energy barrier and require photoexcitation to insert into the H-H bond to form MH$\sb2.$ Bond angles for the dihydrides (MH$\sb2)$ of the above metals have been estimated through measured relative intensities of symmetric and asymmetric stretching modes. We have found that CaH$\sb2$ and ZnH$\sb2$ are linear (or near linear) molecules, while ScH$\sb2,$ TiH$\sb2,$ VH$\sb2,$ CrH$\sb2,$ MoH$\sb2,$ GaH$\sb2$ and GeH$\sb2$ are bent. The stretching force constants for MH$\sb2$ have been calculated with a diatomic approximation and are found to increase across the row (from Ca to Zn). A correlation between the stretching force constants and the size of the metals and the involvement of 3d electrons in bonding appears to exist. The bent nature of the transition metal dihydrides is also thought to be a consequence of involvement of 3d electrons in bonding. Monohydrides of V, Mo, Ga, and trihydrides of Sc, Cr, Mo have also been isolated as results of photoreactions of the respective metals with H$\sb2.$ Formation of polyhydrides (MH$\sb{\rm n},$ n = 4, 6, etc.) was also observed in the reactions of atomic Ti, and Mo with H$\sb2.$ Reactions of metal dimers and trimers with hydrogen have also been observed. Species with terminal hydrogens resulting from the insertion of the dimers of Ca, Sc, Ti, Cr, Ga and Ge into the H-H bond have also been identified as the reaction products of photolysis. Spontaneous reactions of Ga$\sb2$ with H$\sb2$ yield Ga$\sb2$H$\sb2$ with two bridging hydrogens. The trimer of gallium is thought to be isolated in Ar matrices and found to react with H$\sb2$ to form Ga$\sb3$H$\sb2$ with two terminal hydrogens upon absorption of UV light. A ligand-free molecular coordinated dihydrogen complex with the calcium dimer, $\rm Ca\sb2(H\sb2)\sb2,$ was also observed in Kr matrices. Photolysis with UV light produces methylgallium hydride, CH$\sb3$GaH, in the reactions of gallium with methane in Ar, Kr and pure methane matrices