Solid-state spectroscopic properties and the geometry of binuclear rhodium(I) diisocyanoalkane complexes

A series of crystalline dinuclear rhodium complexes with different bridging diisocyano ligands and different counter ions have been studied by low-temperature crystallographic and solid-state spectroscopic techniques. The Rh-Rh distances vary from 4.5153(3) to 3.0988(7) Å, and the twist angles around the Rh-Rh line from 58.3(1) to 0°, both depending on the size and conformational rigidity of the bridging ligand. For very long distances as occur in the [Rh₂(dimen)₄]²⁺ salts the absorption is significantly blue-shifted compared to other complexes. For a given cation a shorter Rh-Rh bond gives a red shift of the phosphorescence emission band, indicating a smaller energy gap between the ground and emitting excited states. An exception occurs for the [Rh₂(1,6-diisocyanohexane) ₄]²⁺ ion, in which dimer formation in the calixarate salt lengthens the Rh-Rh intramolecular bond length without affecting the emission spectrum.