Block-localized wave function (BLW) method, which is a variant of the ab initio valence bond (VB) theory, was employed to explore the nature of resonance-assisted hydrogen bonds (RAHBs) and to investigate the mechanism of synergistic interplay between delocalization and hydrogen-bonding interactions. We examined the dimers of formic acid, formamide, 4-pyrimidinone, 2-pyridinone, 2-hydroxpyridine, and 2-hydroxycyclopenta-2,4-dien-1-one. In addition, we studied the interactions in -diketone enols with a simplified model, namely the hydrogen bonds of 3-hydroxypropenal with both ethenol and formaldehyde. The intermolecular interaction energies, either with or without the involvement of resonance, were decomposed into the Hitler-London energy (DE HL ), polarization energy (DE pol ), charge transfer energy (DE CT ), and electron correlation energy (DE cor ) terms. This allows for the examination of the character of hydrogen bonds and the impact of conjugation on hydrogen bonding interactions. Although it has been proposed that resonance-assisted hydrogen bonds are accompanied with an increasing of covalency character, our analyses showed that the enhanced interactions mostly originate from the classical dipoledipole (i.e., electrostatic) attraction, as resonance redistributes the electron density and increases the dipole moments in monomers. The covalency of hydrogen bonds, however, changes very little. This disputes the belief that RAHB is primarily covalent in nature. Accordingly, we recommend the term ''resonance-assisted binding (RAB)'' instead of ''resonance-assisted hydrogen bonding (RHAB)'' to highlight the electrostatic, which is a long-range effect, rather than the electron transfer nature of the enhanced stabilization in RAHBs.
Keywords: Lithiation / Ligand design / Schiff bases / Imines / Coordination polymer ortho-Metallated imines are commonly used as ligands for late transition metals. Unfortunately, not all metals, such as titanium, zirconium, and niobium, can undergo the necessary oxidative addition reactions to form the desired ortho-metallated complexes directly. Therefore, a synthetic methodology allowing easy access to this binding mode from simple early transition metal halides via an ortho-lithiated imine precursor is desirable. ortho-Lithiation of benzylamines and other systems has been well studied; in contrast, that of imines is poorly developed. However, inclusion of a 3,4-methylenedioxy group on phenyl imines allows for straightforward lithiation and simple isolation of the ortho-lithiated imines. NMR spectroscopy and single-crystal X-ray diffraction allowed for the structural elucidation of the clustering in these lithium complexes. It has been determined that the nature of the im-
A series of niobium and tantalum imido complexes with mono-anionic ortho-metallated acetophenone imine ligands have been prepared and characterized using NMR spectroscopy, mass spectrometry and elemental analysis. These low symmetry complexes are produced with only one or two structural isomers in all cases and display interesting correlations between the steric bulk of the ligands employed and the isomers formed. Crystal structures of several new niobium and tantalum complexes are presented as confirmation of the connectivity in these structural isomers.
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