A DFT based investigation into the electronic structure and properties of hydride rich rhodium clusters

Abstract

Density functional theory has been used to investigate the structures, bonding and properties of a family of hydride rich late transition metal clusters of the type [Rh₆(PH₃)₆H₁₂]^x (x = 0, +1, +2, +3 or +4), [Rh₆(PH₃)₆H₁₆]^x (x = +1 or +2) and [Rh₆(PH₃)₆H₁₄]^x (x = 0, +1 or +2). The positions of the hydrogen atoms around the pseudo-octahedral Rh₆ core in the optimized structures of [Rh₆(PH₃)₆H₁₂]^x (x = 0, +1, +2, +3 or +4) varied depending on the overall charge on the cluster. The number of semi-bridging hydrides increased (semi-bridging hydrides have two different Rh–H bond distances) as the charge on the cluster increased and simultaneously the number of perfectly bridging hydrides (equidistant between two Rh centers) decreased. This distortion maximized the bonding between the hydrides and the metal centers and resulted in the stabilization of orbitals related to the 2T_2g set in a perfectly octahedral cluster. In contrast, the optimized structures of the 16-hydride clusters [Rh₆(PH₃)₆H₁₂]^x (x = +1 or +2) were similar and both clusters contained an interstitial hydride, along with one terminal hydride, ten bridging hydrides and two coordinated H₂ molecules which were bound to two rhodium centers in an η²:η¹-fashion. All the hydrides were on the outside of the Rh₆ core in the lowest energy structures of the 14-hydride clusters [Rh₆(PH₃)₆H₁₄] and [Rh₆(PH₃)₆H₁₄]⁺, which both contained eleven bridging hydrides, one terminal hydride and one coordinated H₂ molecule. Unfortunately, the precise structure of [Rh₆(PH₃)₆H₁₄]²⁺ could not be determined as structures both with and without an interstitial hydride were of similar energy. The reaction energetics for the uptake and release of two molecule of H₂ by a cycle consisting of [Rh₆(PH₃)₆H₁₂]²⁺, [Rh₆(PH₃)₆H₁₆]²⁺, [Rh₆(PH₃)₆H₁₄]⁺, [Rh₆(PH₃)₆H₁₂]⁺ and [Rh₆(PH₃)₆H₁₄]²⁺ were modelled, and, in general, good agreement was observed between experimental and theoretical results. The electronic reasons for selected steps in the cycle were investigated. The 12-hydride cluster [Rh₆(PH₃)₆H₁₂]²⁺ readily picks up two molecules of H₂ to form [Rh₆(PH₃)₆H₁₆]²⁺ because it has a small HOMO–LUMO gap (0.50 eV) and a degenerate pair of LUMO orbitals available for the uptake of four electrons (which are provided by two molecules of H₂). The reverse process, the spontaneous release of a molecule of H₂ from [Rh₆(PH₃)₆H₁₆]⁺ to form [Rh₆(PH₃)₆H₁₄]⁺ occurs because the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital in [Rh₆(PH₃)₆H₁₆]⁺ is 0.9 eV, whereas in [Rh₆(PH₃)₆H₁₄]⁺ the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital is only 0.3 eV. At this stage the factors driving the conversion of [Rh₆(PH₃)₆H₁₄]⁺ to [Rh₆(PH₃)₆H₁₂]²⁺ are still unclear.