Infrared spectra of the WH4(H2)4 complex in solid hydrogen

The codeposition of laser-ablated tungsten atoms with neat hydrogen at 4 K forms a single major product with a broad 2500 cm(-1) and sharp 1860, 1830, 1782, 1008, 551, and 437 cm(-1) absorptions, which are assigned to the WH4(H-2)(4) complex on the basis of isotopic shifts and agreement with isotopic frequencies calculated by density functional theory. This D-2d structured complex was computed earlier to form exothermically from W atoms and hydrogen molecules. Annealing the matrix allows hydrogen to evaporate and the complex to aggregate and ultimately to decompose. Comparison of the H-H stretching mode at 2500 cm(-1) and the W-H-2 stretching mode at 1782 cm(-1) with 2690 and 1570 cm(-1) values for the Kubas complex W(CO)(3)(PR3)(2)(H-2) suggests that the present physically stable WH4(H-2)(4) complex has more strongly bound dihydrogen ligands. Our CASPT2 calculations suggest a 15 kcal/mol average binding energy per dihydrogen molecule in the WH4(H-2)(4) complex.