Easily Broken Strong Bonds: a New Law of Thermodynamics

Abstract

Determination of the molecular structures of tetrakis[bis-(trimethylsilyl)methyl]diphosphine and its arsenic analog in the crystalline phase showed that the central P-P and As-As bonds were little longer than in other diphosphines and diarsines. Nevertheless, their vapors consisted entirely of the bis[bis(trimethylsilyl)methyl]phosphido and bis[bis(trimethylsilyl)methyl]arsenido free radicals. This thermodynamic conundrum has been solved by series of ab initio calculations, which have shown that the energy needed to break the strong central bonds is stored in deformed ligands. Detailed analysis of the dissociation process, aided by a simple ball and spring model, has allowed the intrinsic energy of the central bond to be separated from the overall dissociation energy. The intrinsic bond energies are consistent for a wide range of disphosphines, even though their dissociation energies are scattered over a range of hundreds of kJ mol-1.