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

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journal contribution
posted on 31.01.1996, 00:00 by Buyong Ma, Charlene L. Collins, Henry F. Schaefer
Ab initio quantum mechanical methods were employed to study the periodic trends of transition metal (M = Ti, V, and Cr) hydrides MH2, dihydride dihydrogen complexes MH2·H2, and tetrahydrides MH4. 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 TiH2 and VH2 were found to be 3B1 and 4B2, respectively. The bond angles for the TiH2 and VH2 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 TiH2, VH2, and CrH2 molecules are 1A1, 2A1, and 3B2, 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 TiH2, VH2, and CrH2 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 MH2·H2 complexes. All three MH2·H2 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 H2 moiety more strongly than do the high-spin species. The d → σ* back donation was so strong for the low-spin TiH2 that H2 dissociates without barrier upon contact with singlet TiH2 to form TiH4. Due to the Jahn−Teller distortion the ground state of VH4 is the 2A1 electronic state having D2d symmetry. TiH4 is predicted to lie 9 kcal mol-1 lower in energy than its ground state MH2·H2 isomer, whereas VH4 and CrH4 are higher in energy by 22 and 39 kcal mol-1, respectively, at the TZP CCSD level of theory. However, comparing MH4 and MH2·H2 in the same spin state, MH4 is always lower in energy than its dihydrogen complex isomer, MH2·H2, 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 H2 molecules confirmed the existence of CrH2·H2 by identifying a strong unique absorption at 1510 cm-1. It was found TiH2·H2 rather than TiH2 may be observed experimentally, and that VH2·H2 may be formed concomitantly with the VH2 molecule.