Magnesium dicyanide: Three isomers or seven?

We present a detailed study of the singlet potential energy surface for Mg( CN)(2) using a variety of ab initio computational techniques. When second-o rder Moller-Plesset perturbation theory is employed in conjunction with bas is sets of various sizes, seven structures for Mg(CN)2 are identified as lo cal minima: the linear isomers NCMgCN, NCMgNC, and CNMNC and the pi-complex species NCMg-pi-(CN), CNMg-pi-(CN), and Mg[-pi-(CN)](2) (two enantiomers). These isomers are connected by eight transition states to isomerization. H owever, while the linear structures are also found to be minima at all of t he levels of theory employed here, the existence of the pi-complexes (and, consequently, of many of the transition states) is strongly level-dependent : at B3-LYP/6-31+G*, I33-LYP/6-311+G(2df), and with Hartree-Fock calculatio ns with a variety of basis sets, none of the pi-complexes correspond to sta tionary points upon the potential energy surface. Furthermore, calculations employing methods designed to deliver highly accurate molecular energies ( such as G2 and CBS-Q) reveal that the pi-complexes located on the MP2/6-31G * surface are higher in energy than some of the putative transition states leading to linear isomers. While a more detailed examination of partially o ptimized structures upon the potential energy surface (using various levels of theory including QCISD/6-311G(2df), G2, and CBS-Q, with B3-LYP/6-311+G( 2df) geometries) suggests that the pi-complexes are, technically, local min ima, we conclude that these pi-complexes are, at best, highly reactive inte rmediates on the isomerization pathways NCMgCN <-> NCMgNC and NCMgNC <-> CN MgNC and that only the linear minima (NCMgCN, NCMgNC, and CNR/IgNC) corresp ond to meaningful and isolable chemical entities. According to both the G2 and CBS-Q techniques, the difference between the highest transition state a nd the global minimum (CNMgNC) is only similar to 30 kJ mol(-1).