Revisiting the Synthesis and Elucidating the Structure of Potassium Cyclopentadienyldicarbonylruthenate, K[CpRu(CO)2]

Known procedures for the synthesis of K­[CpRu­(CO)2] (KRp) via reductive cleavage of the ruthenium dimer Rp2 were found to be inconsistent and have thus been revisited, and a revised protocol using K­[HB­(sec-Bu)3] (K-Selectride) as the reducing agent is now reported that gives yellow KRp in crystalline form in around 40% yield. The structure of KRp·THF has been determined by X-ray diffraction, representing the first crystallographic characterization of an Rp– salt. Inevitably the reductive cleavage of Rp2 also gives a poorly soluble black solid as an additional product, which has now been analyzed by a variety of methods, including 13C MAS NMR spectroscopy using 13CO-labeled material. The black solid has been identified as a polymeric Cp/Ru/CO compound with both bridging and terminal CO ligands in a 3:1 ratio. The present report may stimulate the use of the [CpRu­(CO)2]− (Rp–) anion, which has been barely exploited as yet in comparison to its popular congener [CpFe­(CO)2]− (Fp–).