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 2, dihydride dihydrogen complexes MH 2 ·H 2, and tetrahydrides MH 4 . The configuration interaction with single and double excitations (CISD), coupled cluster including all single and double substitutions (CCSD) methods, and CCSD with the effects of connected triple excitation added perturbatively [CCSD(T)] were used with the TZP, TZP+f, and TZP(f,d) basis sets. The ground electronic states for TiH 2 and VH 2 were found to be 3 B 1 and 4 B 2, respectively. The bond angles for the TiH 2 and VH 2 molecules are predicted to be 142° and 139°, respectively, at the TZP(f,d) CISD level of theory. On the low-spin potential energy surfaces, the lowest lying electronic states for the TiH 2, VH 2, and CrH 2 molecules are 1 A 1, 2 A 1, and 3 B 2, respectively. The energy separations between the ground state and the lowest lying low-spin state were found to be 33, 40, and 59 kcal mol -1 for the TiH 2, VH 2, and CrH 2 molecules, respectively, at the TZP CCSD level of theory. The binding energies of the dihydride dihydrogen complexes decrease with increasing atomic number. The d → σ* back donation dominates the periodic trend for the formation of low-spin MH 2 ·H 2 complexes. All three MH 2 ·H 2 complexes are in the high-spin ground state, primarily due to the fact that the corresponding parent dihydrides have high-spin ground states. The low-spin dihydrides interact with the H 2 moiety more strongly than do the high-spin species. The d → σ* back donation was so strong for the low-spin TiH 2 that H 2 dissociates without barrier upon contact with singlet TiH 2 to form TiH 4 . Due to the Jahn−Teller distortion the ground state of VH 4 is the 2 A 1 electronic state having D 2 d symmetry. TiH 4 is predicted to lie 9 kcal mol -1 lower in energy than its ground state MH 2 ·H 2 isomer, whereas VH 4 and CrH 4 are higher in energy by 22 and 39 kcal mol -1, respectively, at the TZP CCSD level of theory. However, comparing MH 4 and MH 2 ·H 2 in the same spin state, MH 4 is always lower in energy than its dihydrogen complex isomer, MH 2 ·H 2, on the low-spin potential energy surface. Comparison between the present work and experimental IR spectra from the matrix isolation of the cocondesation of transition metal atoms (Ti, V, and Cr) with H 2 molecules confirmed the existence of CrH 2 ·H 2 by identifying a strong unique absorption at 1510 cm -1 . It was found TiH 2 ·H 2 rather than TiH 2 may be observed experimentally, and that VH 2 ·H 2 may be formed concomitantly with the VH 2 molecule.