Characterizing Complexes with F-Li...N, H-Li...N, and CH3Li.. Lithium Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants
Ab initio calculations have been carried out to determine the structures, binding energies, and spin−spin coupling constants of complexes stabilized by X−Li···N bonds with F−Li, H−Li, and CH3Li as the Lewis acids. Complexes of these acids with the nitrogen bases N2, HCN, 1,3,5-triazine, pyrazine, 1,2,3-triazine, pyridine, and NH3 have linear X−Li···N bonds. Methylamine forms a nonlinear lithium bond only when F−Li is the lithium donor. Two bases, HN═CH2 and aziridine, form nonlinear X−Li···N bonds with each acid. Except for complexes with N2, which have small binding energies of about 5 kcal/mol, the binding energies of lithium-bonded complexes are appreciable, varying between 15 and 23 kcal/mol. The one-bond coupling constant 1J(F−Li) may increase or decrease upon complexation, but 1J(H−Li) and 1J(C−Li) decrease significantly. These coupling constants have their smallest values in complexes with nonlinear X−Li···N bonds. No correlations appear to exist between 1J(X−Li) and the X−Li distance and 1liJ(Li−N) and the Li−N distance. Values of the two-bond coupling constants 2liJ(X−N) are extremely small. Comparisons of 2liJ(F−N) with 2hJ(F−N) for coupling across a hydrogen bond and 2xJ(F−N) for coupling across a halogen bond suggest that the extremely small values of 2liJ(X−N) are not due to long X−N distances but to the low valence electron density on Li in lithium-bonded complexes.