Do branched structures exist for cyanide-containing magnesium compounds? Computational studies on a range of mixed-ligand compounds XMg(CN) (X = F, Cl, OH, SH, NH2, CH3, CN)

Cyanide compounds of the alkali metals and alkaline earths are commonly fou nd to possess "branched" or pi-complex structures in which the metal atom i s almost equidistant from both atoms of the CN moiety. Here we present an i nvestigation of the potential energy surfaces for various compounds of the form XMg(CN), using the Gaussian-2 (G2) procedure. Our results suggest that magnesium, at least, is not so prone to pi-complex formation with the cyan ide ligand as has previously been implied, since the presence of the pi com plex upon the potential energy surface is strongly dependent upon the level of theory employed in geometry optimizations. We find also that, according to G2 theory, the preference of magnesium for isocyanide (rather than cyan ide) formation is small but consistent, with XMgNC isomers having calculate d heats of formation between 2 and 5 kJ mol(-1) below their XMgCN counterpa rts. The barriers to interconversion of cyanide and isocyanide isomers are also calculated to be comparatively small, typically similar to 25 kJ mol(- 1). In contrast, calculations for protonated species FMg(CN)H+ and Mg(CN)(2 )H+ have determined that the rr complexes in these species are indeed stabl e against CN-ligand reorientation. (C) 2000 John Wiley & Sons, Inc.