MNgCCH (M = Cu, Ag, Au; Ng = Xe, Rn): The First Set of Compounds with M-Ng-C Bonding Motif.

Although Ng-M (M = Cu, Ag, Au; Ng = noble gas) and Ng-C bonds are known to exist in different viable species, we report here a series of systems with formula MNgCCH (Ng = Xe, Rn) in which both bonds coexist. These compounds possess reasonably high kinetic stability (free energy barrier, ΔG‡ of 14.0-34.8 kcal/mol) along an exergonic isomerization channel, MNgCCH → NgMCCH. For a given M, the ΔG‡ associated with this channel increases from Xe to Rn, whereas for a given Ng, it increases along Ag < Cu < Au. No other possible dissociation channel is feasible at standard condition, except for the Ag-Xe analogue, where one three-body neutral dissociation channel, AgXeCCH → Ag + Xe + CCH, is slightly exergonic by 2.4 kcal/mol. Examination of the thermochemical stability of the Ng-M bonds in noninserted compounds against the dissociation, NgMCCH → Ng + MCCH reveals that Kr-Rn bound Cu and Au analogues, and Xe and Rn bound Ag analogues would be viable at 298 K. The natural bond order analysis indicates the formation of M-Ng covalent bond and Ng-C ionic bonds in these compounds having an ionic representation of (MNg)+(CCH)-. Energy decomposition analysis reveals a significant contribution of the electrostatic term in the M-Ng covalent bonds.