THERMOCHEMISTRY OF GASEOUS METHANIDE, C4- AND ETHYNIDE, C-2(2-) IONS

Ionic models adopted for the methanides, Be2C and Al4C3 lead to the ca lculation of the enthalpy of formation of the gaseous anion, Delta(f)H degrees(C4-, g)/kJ mol(-1) = 6018+/-200 and of the total lattice pote ntial energies of these salts to be: 12218+/-150 and 39790+/-250 respe ctively. For the corresponding ethynide anion, a value of 920+/-8 kJ m ol(-1) is assigned to Delta(f)H degrees(C-2(2-), g). A thermochemical radius/nm of 0.161 nm found for the C4- anion and the corresponding va lue of 0.185 nm for the C-2(2-) anion is utilised to predict the latti ce energies U-POT/kJ mol(-1) of MgC2 (=3209+/-30), Li2C2 (=2401+/-24) and Na2C2 (=2260+/-20). Ehthalpies for the processes: C4- (g) --> C- ( g) + 3e; C-2(2-) (g) --> 2 C- (g); C-2(2-) (g) --> C-2(+) (g) + 3e and C2- (g) + 6e --> 2 C4- (g) are assigned to be: -5429+/-153 kJ mol(-1) (-56.2+/-1.6 eV), 1084+/-35 kJ mol(-1) (11.2+/-0.4 eV), 257+/-8 kJ mo l(-1) (2.7+/-0.1 eV) and 11116+/-216 kJ mol(-1) (115+/-2 eV) respectiv ely.