Synthesis, structure determination, and quantum-chemical characterization of an alternate HgNCN polymorph

A novel, possibly metastable form of HgNCN, designated HgNCN(II), is accessible under soft-chemical reaction conditions, and its existence has been established from combined X-ray/neutron Rietveld refinements (P2(1)/a, a = 6.8521(4) A, b = 6.9797(4) A, c = 5.5516(4) A, beta = 113.212(4) degrees ), vibrational spectroscopy investigations, and plane-wave DFT calculations utilizing pseudopotentials. In contrast to the known mercury carbodiimide HgNCN(I) with two practically identical N-C double bonds of 1.22 A, the true cyanamide HgNCN(II) is characterized by a short ("triple") N-C bond (1.12 A) and a long ("single") C-N bond (1.35 A); two Hg atoms coordinate the asymmetrically shaped NCN(2)(-) unit only on the side of the C-N "single" bond. Mercury carbodiimide HgNCN(I) and mercury cyanamide HgNCN(II) are thermochemically separated from each other by such a high energy barrier that thermal decomposition prevents structural interconversion. The structure-chemical relationship of the two HgNCN phases is discussed in terms of bond-stretch and linkage isomerism.