Research on zero-valent p-block elements is a recent hot topic in synthetic and theoretical chemistry because of their novel electronic states having two lone pairs in both the s- and p-orbitals. It is considered that σ-donating ligands bearing large substituents are essential to stabilize these species. Herein, we propose a new approach using butadiene as a 4π-electron donor to stabilize zero-valent group 14 elements. During our study to explore the coordination chemistry of stannacyclopentadienyl ligands, unexpected products, in which the tin atom is coordinated by a butadiene in a η(4)-fashion, were obtained. Because butadiene is a neutral 4π-electron donating ligand, the formal oxidation number of the tin atoms of the products should be zero, which is supported by X-ray diffraction analysis and theoretical calculations. A mechanism for the formation of the products is also described.

Reduction of THF-stabilized plumbacyclopentadienylidene with lithium afforded dilithioplumbole. On the other hand, oxidation of the dilithioplumbole provided the starting plumbacyclopentadienylidene. This is the unprecedented example of a reversible interconversion between group 14 M(II) and its dianionic species bearing organic substituents.

A new possibility of a Rh(I) complex is demonstrated. An inverted sandwich Rh(I) complex bearing a dianionic plumbole ligand was synthesized by the reaction a dilithioplumbole with [Rh(cod)Cl] 2 . The plumbole ligand functions as a π-donor stronger than the cyclopentadienyl ligand, as evidenced by quantum-chemical calculations. The Rh(I)−plumbole complex can be applied as a catalyst for an intramolecular [2+2+2] cycloaddition reaction of a triyne.

An anionic ruthenocene with an η5-coordinating plumbole ligand was prepared and reacted with electrophiles to afford plumbole complexes.