text: Adding to the transition metal-bismuth carbonyl clusters, the dibismuth clusters with transition metals have also been explored by synthetic chemists. The core of such compounds is represented in the form of dibismuthene or dibismithyne unit, in which the Bi atoms contain the inert 6s lone pair and through π-bond-donation are able to coordinate to carbonyl moieties of transition metals . 
  The common synthetic precursor is the trimethylsilylmethyl-cyclobismuthane. Upon reaction with tungsten pentacarbonyl, the resulting side-on adduct preserved the dibismuthene unit, while reaction with diiron noncarbonyl yields the a tetracylic heteronuclear iron-bismuth carbonyl compound (see scheme to the right).
  The complexity of the dibismuthene complexes ranges from incorporation of cobalt ions to generate a prismatic cobalto carbonyl dicapped structure in the [(CO)CoBi] structure to iron incorporation to yield diiron dibismuth tetracyclic moiety side-on capped with cobaltocarbonyl unit. A similar structure was synthesized with tungsten replacing the iron units and this time capped with a bismuth-iron carbonyl-Cp unit. Finally, another example comes in the form of a side-on coordinated zirconium dicyclopentadienyl unit to the dibismuth mesitylene moiety (see figure).