{μ-PbSe}: a heavy CO homologue as an unexpected ligand.

Reactions of [K(18-crown-6)]2 [Pb2 Se3 ] and [K([2.2.2]crypt)]2 [Pb2 Se3 ] with [Rh(PPh3 )3 Cl] in en (ethane-1,2-diamine) afforded ionic compounds with [Rh3 (PPh3 )6 (μ3 -Se)2 ](-) and [Rh3 (CN)2 (PPh3 )4 (μ3 -Se)2 (μ-PbSe)](3-) anions, respectively. The latter contains a PbSe ligand, a rather uncommon homologue of CO that acts as a μ-bridge between two Rh atoms. Quantum chemical calculations yield a significantly higher bond energy for PbSe than for CO, since the size of the ligand orbitals better matches the comparably rigid Rh-Se-Rh angles and the resulting Rh⋅⋅⋅Rh distance. To rationalize the bent coordination of the ligand, orbitals with significant ligand contributions and their dependence on the bonding angle were investigated in detail.