{μ-PbSe}: A Heavy CO Homologue as an Unexpected Ligand

Reactions of [K(18-crown-6)](2)[Pb2Se3] and [K-([ 2.2.2] crypt)](2)[Pb2Se3] with [Rh(PPh3)(3)Cl] in en (ethane-1,2-diamine) afforded ionic compounds with [Rh-3(PPh3)(6)(mu(3)-Se)(2)](-) and [Rh-3(CN)(2)(PPh3)(4)(mu(3)-Se)(2)(mu-PbSe)](3-) anions, respectively. The latter contains a PbSe ligand, a rather uncommon homologue of CO that acts as a m-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.