Matrix Infrared Spectroscopic and Theoretical Studies for the Products of Lead Atom Reactions with Ethane and Halomethanes.

The insertion products of laser-ablated Pb atom reactions with ethane and mono-, di-, tri-, and tetrahalomethanes in excess argon were prepared and identified from their matrix infrared spectra on the basis of DFT computed frequencies and observed isotopic shifts. Unlike the lighter elements in group 14, the heaviest member lead exists primarily in the oxidation state 2+ using 6p orbitals in reaction products due to relativistic contraction of the 6s orbital. The C-Pb-X (X = H, F, Cl) bond is close to a right angle, indicating that Pb contributes mostly p-character to the C-Pb and Pb-X bonds. The lead reaction product with ethane is CH3CH2-Pb-H. The lower energy product in the CH2FCl reaction is CH2F-PbCl, which is photoisomerized to CH2Cl-PbF. A lead methylidene (CCl2-PbCl2) was identified only in reactions with CCl4. The relatively small energy difference between the insertion and methylidene products in the tetrachloride system allows photochemical conversion from the insertion product to the unusual 3+ Pb oxidation-state methylidene. In this case Pb uses three p orbitals in bonding and C is sp2 hybridized leaving spin paired but not bonding by symmetry single electrons in the C 2p orbital perpendicular to the CCl2 plane and in the Pb 6s orbital. More often lead uses 6p orbitals in bonding due to the high electronic promotion energy for 6s electrons.