Periodic Trends for Transition Metal Dihydrides MH2, Dihydride Dihydrogen Complexes MH2·H2, and Tetrahydrides MH4 (M = Ti, V, and Cr)

Abstract

Ab initio quantum mechanical methods were employed to study the periodic trends of transition metal (M = Ti, V, and Cr) hydrides MH{sub 2}, dihydride dihydrogen complexes MH{sub 2}.H{sub 2}, and tetrahydrides MH{sub 4}. All three MH{sub 2}.H{sub 2} complexes are in the high-spin ground state. The low-spin dihydrides interact with the H{sub 2} moiety more strongly than do the high-spin species. The d {yields} {sigma}{sup *} back donation was so strong for the low-spin TiH{sub 2} that H{sub 2} dissociates without barrier upon contact with singlet TiH{sub 2} to form TiH{sub 4}. Due to the Jahn-Teller distortion the ground state of VH{sub 4} is the {sup 2}A{sub 1} electronic state having D{sub 2d} symmetry. TiH{sub 4} is predicted to lie 9 kcal mol{sup -1} lower in energy than its ground state MH{sub 2}.H{sub 2} isomer, whereas VH{sub 4} and CrH{sub 4} are higher in energy by 22 and 39 kcal mol{sup -1}, respectively, at the TZP CCSD level of theory. However, comparing MH{sub 4} and MH{sub 2}.H{sub 2} in the same spin state, MH{sub 4} is always lower in energy than its dihydrogen complex isomer, MH{sub 2}.H{sub 2}, on the low-spin potential energy surface. Comparison between the present workmore » and experimental IR spectra from the matrix isolation of the cocondensation of transition metal atoms (Ti, V, and Cr) with H{sub 2} molecules confirmed the existence of CrH{sub 2}.H{sub 2} by identifying a strong unique absorption at 1510 cm{sup -1}. 17 refs., 4 figs., 8 tabs.« less