Ligand fluxionality in close-packed rhodium carbonyl clusters

Abstract

A combination of variable-temperature, multinuclear n.m.r. studies (¹H, ¹³C, ¹⁰³Rh, ¹H-{¹⁰³Rh}, and ¹³C-{¹⁰³Rh}) has allowed the ligand (CO/H) migrations in [Rh₁₃(CO)₂₄H_n]^{(5 –n)–}(n= 1–4) and [Rh₁₄(CO)₂₅H_X]^{(4 –x)–}(x= 0 or 1) to be established. In both types of compound, interstitial H migration is found at room temperature with complete randomisation of carbonyls in the Rh₁₄ clusters and partial migration of carbonyls in the Rh₁₃ clusters; in neither case is there evidence for rearrangement of the metal polyhedron. At low temperatures (–90 °C), H migration still occurs in [Rh₁₄(CO)₂₅H]^3– but not in [Rh₁₃(CO)₂₄H₃]^2– and [Rh₁₃(CO)₂₄H₂]^3– which have the same structural features as found in the solid state with the hydride ligands occupying pseudooctahedral holes in the metal skeleton. All the values of δ(¹⁰³Rh) for the interstitial rhodium atoms are at significantly higher frequency than the values for the surface rhodium resonances and shift significantly to lower frequency with increasing interstitial hydride content.