Experimental and quantum-chemical studies on the thermochemical stabilities of mercury carbodiimide and mercury cyanamide.

Calorimetric dissolution measurements of the solid compounds mercury carbodiimide HgNCN(I) and mercury cyanamide HgNCN(II) in aqueous HCl that targeted at their thermochemical stabilities show the cyanamide species HgNCN(II) to be the more stable phase in terms of both enthalpy and Gibbs energy with an enthalpy difference of 2-3 kJ mol-1. While the stability ranking of HgNCN(I) and HgNCN(II) thus perfectly matches Pearson's HSAB concept, quantum-chemical stability predictions using common parametrizations of density functional theory appear to be fundamentally flawed. An analysis of the error is attempted on the basis of correlated wave functions for related molecules.