Reducing and Reversing the Diphosphene-Diphosphinylidene Energy Separation.

The dependence of the relative energies of 116 diphosphene and diphosphinylidene compounds on the modification of their structures is studied theoretically. Optimized geometries and relative energies are reported for all structures. With the purpose of investigating the effects of various substituents on the parent PPH2 and HPPH molecules, isodesmic reaction energies were obtained for single and double substitution. In the case of the substitution of both H atoms by lithoxy (OLi) or ONa groups is the diphosphinylidene type structure found to be lower in energy. For the lithoxy group, the energy difference amounts to 33 kcal/mol at CCSD(T) cc-pVTZ level of theory. This result is explained through the natural population analyses, where a very favorable Coulombic attraction is found in the OLi substituted diphosphinylidene structure. The order of the effectiveness of the substituents in lowering the relative energy of the diphosphinylidene structure is OLi > ONa > OH > OSiH3 > OCH3 > OPh > NH2 > N(CH3)2 > F > ONH2 > OBH2 > CH3 > OOH > Ph > BF2 > PH2 > SiH3 > SH > HC═O > Cl > CF3 > Br > SiF3 > NF2 > NO2 > C≡CH > OF > CN. Natural bond orbital (NBO) analysis explains other qualitative bonding features, for example, phosphorus-phosphorus bond orders as large as 2.5 for R2PP structures and as small as 1.6 for RPPR structures.