Synthesis and structural characterization of a diruthenium pentalene complex, $$[\hbox {Cp}^{*}\hbox {Ru}\{(\hbox {Cp}^{*}\hbox {Ru})_{2}\hbox {B}_{6}\hbox {H}_{14}\}(\hbox {Cp}^{*}\hbox {Ru})]$$[Cp∗Ru{(Cp∗Ru)2B6H14}(Cp∗Ru)]