Structural Trends in the Monocyanides of the Second-Row Metal Ions Na+, Mgm+ (m = 1, 2), and Aln+ (n = 1−3)

Abstract

The G2 (Gaussian-2) theoretical procedure has been used to determine equilibrium geometries, M−(CN) and Mj+−(CN-) bond energies, total energies, and isomerization pathways of the cyanide and isocyanide isomers of the species Na+(CN-), Mgm+(CN-) [m = 1, 2], and Aln+(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-Al(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 bon...