The nature of the phosphorus–tellurium bond in tertiary phosphine tellurides is not well understood. There is also controversy over the nature of multiple bonding in the lighter chalcogenides and the related ylides and imides. Density functional theory (DFT) was used to investigate the interactions in the molecule, Me3PE (E = O, S, Se, Te, BH3, CH2, NH). The calculated PE bond energies and orbital populations reveal contributions from both σ donation from the phosphine and π back-donation to the phosphine in all of the above cases. Down the group from oxygen to tellurium, the PE bond weakens from 544 kJ mol−1 to 184 kJ mol−1, but multiple bonding becomes more significant with respect to the single bond. For E = BH3, the PB bond energy is 166 kJ mol−1. Trimethylphosphine ylide was found to have a π-bond order of 0.5, while that of trimethylphosphine imine is 0.6. For comparison, the oxides of trimethylamine and trimethylarsine were also calculated to examine the pnictogen–oxygen bond; Me3N does not participate in multiple bonding with oxygen, while the π-bond orders for Me3PO and Me3AsO were calculated as 0.7 and 0.6, respectively. Key words: phosphine chalcogenides, phosphine ylides, phosphine imides, DFT calculations
