Carbodiphosphorane analogues E(PPh ₃) ₂ with E=C-Pb: A theoretical study with implications for ligand design

Quantum-chemical calculations at the BP86/TZVPP level have been carried out for the heavy Group 14 homologues of carbodiphosphorane E(PPh ₃) ₂, where E=Si, Ge, Sn, Pb, which are experimentally unknown so far. The results of the theoretical investigation suggest that the tetrelediphosphoranes E(PPh ₃) ₂ (1 E) are stable compounds that could become isolated in a condensed phase. The molecules possess donor-acceptor bonds Ph ₃P→E←PPh ₃ to a bare tetrele atom E, which retains its four valence electrons as two electron lone pairs. The analysis of the bonding situation and the calculation of the chemical reactivity indicate that the molecules 1 E belong to the class of divalent E(0) compounds (ylidones). All molecules 1 C-1 Pb have very large first but also very large second proton affinities, which distinguishes them from the N-heterocyclic carbene homologues, in which the donor atom is a divalent E(II) species that possesses only one electron lone pair. Compounds 1 E are powerful double donors that strongly bind Lewis acids such as BH ₃ and AuCl in the complexes 1 E(BH ₃) _n and 1 E(AuCl) _n (n=1, 2). The bond dissociation energies (BDEs) of the second BH ₃ and AuCl molecules are only slightly less than the BDE of the first BH ₃ and AuCl. The results of this work are a challenge for experimentalists.