Structures and bonding situation of Pb₂X₂ (X = H, F, Cl, Br and I)

Quantum chemical calculations using DFT (BP86) and ab initio methods (MP2, MP4 and CCSD(T)) have been carried out for the title compounds. The nature of the Pb-Pb interactions has been investigated with an energy decomposition analysis. The energy minimum structures of the halogen substituted Pb₂X₂ molecules possess a doubly bridged butterfly geometry A like the parent system Pb₂H₂. The unusual geometry can be explained with the interactions between PbX fragments in the X²Π ground state which leads to one Pb-Pb electron-sharing σ bond and two donor-acceptor bonds between the Pb-X bonds as donor and vacant p(π) AOs of Pb. The energy difference between the equilibrium form A and the linear structure XPb≡PbX (E) which is a second-order saddle point is much higher when X is a halogen atom than for X = H. This is because the a⁴Σ^- ← X²Π excitation energies of PbX (X = F-I) are higher than for PbH. The structural isomers B, D1, D2, E, F1, F2 and G of Pb₂X₂ are no minima on the potential energy surface.