The computation of metal-silyl interaction energies indicates the existence of situations in which the silyl group behaves as a Z-type ligand according to the Green method of covalent-bond classification. There is a scale of relative intrinsic silylicity Π, defined as the ratio of the intrinsic silyl-to-triflate interaction energy of a silyltriflate as a reference compound relative to the silyl-to-metal interaction of given complex, that can reveal in a straightforward manner the propensity of SiR groups to behave chemically as metal-bound "silylium" ions, namely, [SiR ] . Emblematic cases, either taken from the Cambridge Structural Database (CSD) or constructed for the purpose of this study, were also investigated from the viewpoints of extended transition-state natural orbitals for chemical valence (ETS-NOCV) and quantum theory of atoms in molecules (QTAIM) analyses. It is shown in the case of POBMUP-which is the iridium 1,3-bis[(di-tert-butylphosphino)oxy]benzene (POCOP) complex isolated by Brookhart et al.-how slight variations of molecular charge and structure can drastically affect the relative intrinsic silylicity of the SiEt group that is weakly bonded to the hydrido-iridium motif.

High level ab initio calculations predicted a possibility for energetically low-cost (±1 kcal mol−1) change of the bond angle and bond lengths in wide range,from 96.4° to 112.8° and from 0.930 Å to 0.989 Å, respectively.

All water–water contacts in the crystal structures from the Cambridge Structural Database with d OO ≤ 4.0 Å have been found. These contacts were analysed on the basis of their geometries and interaction energies from CCSD(T)/CBS calculations. The results show 6729 attractive water–water contacts, of which 4717 are classical hydrogen bonds (d OH ≤ 3.0 Å and α ≥ 120°) with most being stronger than −3.3 kcal mol−1. Beyond the region of these hydrogen bonds, there is a large number of attractive interactions (2062). The majority are antiparallel dipolar interactions, where the O—H bonds of two water molecules lying in parallel planes are oriented antiparallel to each other. Developing geometric criteria for these antiparallel dipoles (β1, β2 ≥ 160°, 80 ≤ α ≤ 140° and T HOHO > 40°) yielded 1282 attractive contacts. The interaction energies of these antiparallel oriented water molecules are up to −4.7 kcal mol−1, while most of the contacts have interaction energies in the range −0.9 to −2.1 kcal mol−1. This study suggests that the geometric criteria for defining attractive water–water interactions should be broader than the classical hydrogen-bonding criteria, a change that may reveal undiscovered and unappreciated interactions controlling molecular structure and chemistry.