Structural trends in the monocyanides of the second-row metal ions Na+, Mg-m+ (m=1,2), and Al-n+ (n=1-3)

The G2 (Gaussian-2) theoretical procedure has been used to determine equilibrium geometries, M-(CN) and M(j+)-(CN-) bond energies, total energies, and isomerization pathways of the cyanide and isocyanide isomers of the species Na+(CN-), Mg-m+(CN-) [m = 1, 2], and Al-n+(CN-) [n = 1-3]. At the MP2(full)/6-31G* level, a ''T-shaped'' isomer is also found for each of the 24-electron species Na(CN), Mg(CN)(+), and Al(CN)(2+): however, attempted optimization at the QCISD(full)/6-311G** level suggests that T-AI(CN)2+ is not a true minimum. At the G2 level, the linear MCN and MNC geometries are found to be local minima for all species except NaNC, which constitutes a local maximum for rotation of Na+ about the CN- moiety. A very large discrepancy of 162 kJ mol(-1) is noted between the G2 and G2Q bond dissociation energies for Al-NC2+, with the G2Q value appearing the more reliable. The calculations at the G2Q level suggest that AlNC2+ is a thermodynamically stable dication, with substantial covalent character in the Al-N bond. The total energy of AlNC2+ is substantially lower than that of the other linear isomer, AlCN2+: on the other potential energy surfaces, differences in energy between isomers are generally small, supporting the results of earlier studies on some of these systems, which have found the metal/cyanide bonding to be largely electrostatic.