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

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.