We definitively show that the CO stretching response to metal coordination is driven exclusively by π polarization, which quantitatively correlates with π back-donation and changes in CO bond length and frequency.

We recently devised a simple scheme for analyzing on quantitative grounds the Dewar-Chatt-Duncanson donation and back-donation in symmetric coordination complexes. Our approach is based on a symmetry decomposition of the so called Charge-Displacement (CD) function quantifying the charge flow, upon formation of a metal (M)-substrate (S) bond, along the M-S interaction axis and provides clear-cut measures of donation and back-donation charges in correlation with experimental observables [G. Bistoni et al., Angew. Chem., Int. Ed. 52, 11599 (2013)]. The symmetry constraints exclude of course from the analysis most systems of interest in coordination chemistry. In this paper, we show how to entirely overcome this limitation by taking advantage of the properties of the natural orbitals for chemical valence [M. Mitoraj and A. Michalak, J. Mol. Model. 13, 347 (2007)]. A general scheme for disentangling donation and back-donation in the CD function of both symmetric and non-symmetric systems is presented and illustrated through applications to M-ethyne (M = Au, Ni and W) coordination bonds, including an explicative study on substrate activation in a model reaction mechanism.

A gas-phase nitrogen-nitrogen noncovalent interaction has been unveiled in the nitroethane-trimethylamine complex in an environment free from solvent and matrix effects using rotational spectroscopy in supersonic expansion. Different quantum chemical models (NOCV/CD and NBO) agree in indicating that this interaction largely prevails over the C-H⋅⋅⋅O and C-H⋅⋅⋅N hydrogen bonds. Furthermore, a SAPT analysis shows that electrostatic and dispersion interactions play a comparable role in stabilizing the complex. The conformational landscape exploration and stationary points characterization have been performed using state-of-the-art quantum-chemical computations providing significant insights on structure determination.

By combining rotational spectroscopyinsupersonic expansion with the capability of state-of-the-art quantumchemical computations in accurately determining structural and energetic properties,the genuine nature of as ulfur-sulfur chalcogen bond between dimethyl sulfide and sulfur dioxide has been unveiled in agas-jet environment free from collision, solvent and matrix perturbations.ASAPT analysis pointed out that electrostatic S···S interactions play the dominant role in determining the stability of the complex, largely overcoming dispersion and CÀH···O hydrogen-bond contributions.Indeed, in agreement with the analysis of the quadrupole-coupling constants and of the methyl internal rotation barrier,the NBO and NOCV/CD approaches show am arked charge transfer between the sulfur atoms.B ased on the assignment of the rotational spectra for 7i sotopologues,a na ccurate semiexperimental equilibrium structure for the heavy-atom backbone of the molecular complex has been determined, whichis characterized by aS ···S distance (2.947(3) )w ell belowt he sum of van der Waals radii.