Thorium and Uranium Hydride Phosphorus and Arsenic Bearing Molecules with Single and Double Actinide-Pnictogen and Bridged Agostic Hydrogen Bonds.

Abstract

Thorium atoms from laser ablation react with phosphine during condensation in excess argon to produce two new infrared absorptions at 1467.2 and 1436.6 cm-1 near weak bands for ThH and ThH2, which increase on annealing to 25 and 30 K, indicating spontaneous reactions. Analogous experiments with uranium produced two similar bands at 1473.4 and 1456.7 cm-1 above UH at 1423.8 cm-1 and another absorption at 1388.2 cm-1. Electronic structure calculations at the coupled cluster CCSD(T) for Th and density functional theory calculations for U as well as their proximity to other actinide hydride absorptions support assignments of these bands to the simplest molecules HP═ThH2, HP═UH2, and PH2-UH. Arsine gave the analogous products HAs═ThH2, HAs═UH2, and AsH2-UH. The HE═AnH2 molecules (E = P, As; An = Th, U) have strong agostic An-H(E) interactions with H-E-An angles in the range of 60-64°. The calculated agostic bond distances are 9% to 12% longer than terminal single An-H bonds, which suggests that these strong agostic bonds can be considered as bridge bonds since similar relationships are found for the dibridged M2H6 molecules (M = Al, Ga, In). The NBO analysis and the molecular orbitals show the presence of a σ and a π bond for HE═AnH2 molecules that are heavily polarized with most of the density on the P or As.