Magnesium Dicyanide:  Three Isomers or Seven?

Abstract

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-order Møller−Plesset perturbation theory is employed in conjunction with basis sets of various sizes, seven structures for Mg(CN)2 are identified as local minima: the linear isomers NCMgCN, NCMgNC, and CNMgNC and the π-complex species NCMg-π-(CN), CNMg-π-(CN), and Mg[-π-(CN)]2 (two enantiomers). These isomers are connected by eight transition states to isomerization. However, while the linear structures are also found to be minima at all of the levels of theory employed here, the existence of the π-complexes (and, consequently, of many of the transition states) is strongly level-dependent: at B3-LYP/6-31+G*, B3-LYP/6-311+G(2df), and with Hartree−Fock calculations with a variety of basis sets, none of the π-complexes correspond to stationary 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 π-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 optimized 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 π-complexes are, technically, local minima, we conclude that these π-complexes are, at best, highly reactive intermediates on the isomerization pathways NCMgCN ↔ NCMgNC and NCMgNC ↔ CNMgNC and that only the linear minima (NCMgCN, NCMgNC, and CNMgNC) correspond to meaningful and isolable chemical entities. According to both the G2 and CBS-Q techniques, the difference between the highest transition state and the global minimum (CNMgNC) is only ∼30 kJ mol-1.

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)

“…Our study 12 on the isomerism of Mg(CN) 2 revealed several subtleties in the treatment of this compound by the Gaussian‐2 method. These features may be summarized in point form: At the MP2(full)/6‐31G* level of theory, the structure XMg‐π‐(CN) [X = CN‐, (CN)‐π‐, or NC‐] is a local minimum, with an associated transition state (TS 2) leading to the corresponding isonitrile XMgNC.…”

“… a G2 energies for Mg(CN) 2 stationary points have been reported previously in Ref. 12 and are not reproduced here. …”

“…We have also used a variety of other theoretical methods to investigate Mg(CN) 2 isomerism 12 and conclusions drawn from these calculations may also be usefully summarized: Existence of the π complexes as discrete minima is dependent on the level of theory employed in optimizations. The π‐complex geometries are minima in calculations using second‐ or third‐order Møller–Plesset perturbation theory using a variety of basis sets, and in B3‐LYP (hybrid density functional theory) calculations using a 6‐31G* or 6‐311G* basis; these stationary points cannot be located in any Hartree–Fock calculations which we have attempted, nor in B3‐LYP calculations employing diffuse functions [e.g., the 6‐31+G* and 6‐311+G(2df) basis sets]. Calculations using the CBS‐Q technique 21, which is generally held to be of comparable accuracy to G2 22 but which uses a substantially different method of basis set extrapolation, are entirely consistent with the G2 results in that isocyanide coordination is preferred, π complexes are unstable relative to their putative transition states, and the difference in energy between the highest transition state to isomerization and the global minimum (CNMgNC) is only ∼30 kJ mol −1 . Single‐point CBS‐Q calculations, at B3‐LYP/6‐311+G(2df,p) partially optimized geometries in the vicinity of the putative transition state between NCMg‐π‐(CN) and NCMgNC, indicate that the well depth of NCMg‐π‐(CN) at this level of theory is only 0.11 kJ mol −1 . …”

“…Even greater possibilities for isomerism exist upon the M(CN) 2 (M = Be, Mg, Ca, Sr, Ba) potential energy surfaces: a theoretical investigation by Kapp and Schleyer 9 has revealed that these surfaces are littered with low‐lying minima, with, e.g., four metastable isomers of Ca(CN) 2 calculated to lie less than 36 kJ mol −1 above the global minimum, (CN)‐π‐Ca‐π‐(CN). Kapp and Schleyer have also located several π‐complex isomers upon the Mg(CN) 2 potential energy surface 9; a more recent reexamination 12 of this surface at high levels of theory has determined that none of these π complexes (if they even exist as discrete minima) possess potential well depths greater than ∼2 kJ mol −1 .…”

“…In the present work, we investigate the isomerism of several magnesium cyanide derivatives XMg(CN) [X=CH 3 , NH 2 , OH, F,SH, Cl, CN]. With the exception of the dicyanide isomers 9, 12, these species have not been previously studied at high levels of theory, nor has their experimental isolation been reported to date: our results suggest that many of these valence‐satisfied, essentially ionic compounds are potentially of considerable interest as models of nonrigid or “floppy” molecules 13.…”