Neither too Classic nor too Exotic: One-Electron Na⋅B Bond in NaBH3 - Cluster

Delocalization-ratio analysis of 3-center bonding in position-space for closo-boranes and related systems: Approaching the styx picture and beyond

Closo-boron hydrides BnHn 2- (n = 5-12) are a conceptually well understood class of compounds. For these and a few related prototype compounds, both the local and the global picture of 3-center bonding are extracted from position-space quantities based on the electron density and the pair density. For this purpose, three-center delocalization indices between quantum theory of atoms in molecules (QTAIM) atoms in position space are used to develop a consistent set of local bond and triangle, and global cluster delocalization ratios (DRs), which are quantitatively compared with conceptual Γ values derived from the styx code for each cluster. Combination of the cluster DRs with associated effective numbers of skeletal electron sharing (SES) for selected cluster surface edges, triangles, or the whole cluster yields effective styx type values describing the trend and even the size of the conceptual styx codes for closo-boranes BnHn 2- and related systems with increasing cluster size n reasonably well. For nonuniform cluster topologies, the different vertex degrees are shown to cause systematic 3-center wise bond delocalization effects for the associated edges and triangles of different average vertex degrees. Extension of DR analysis beyond the styx type triangular cluster-surface bonding corresponds to a triangulation of multicentric bonding. The cluster-wise results keep indicating consistency with the mixed 2- and 3-center bonding approach. The successfully established chemical meaning of the local edge, triangle, and global cluster DRs and their associated SES values constitutes the basis for systematic investigations of mixed 2- and 3-center bonding scenarios in particular in intermetallic and related (endohedral) cluster compounds in the future.

Classical versus Collective Interactions in Asymmetric Trigonal Bipyramidal Alkaline Metal-Boron Halide Complexes

Collective interactions are a novel type of chemical bond formed between metals and electron-rich substituents around an electron-poor central atom. So far only a limited number of candidates for having collective interactions are reported. In this work, we extend the newly introduced concept of collective bonding to a series of neutral boron complexes with the general formula M2BX3 (M=Li, Na, and K; X=F, Cl, and Br). Our state-of-the-art ab initio computations suggest that these complexes form trigonal bipyramidal structures with a D3h to C3v distortion along the C3 axis of symmetry. The BX3 unit in the complexes distorts from planar to pyramidal akin to a sp3 hybridized atom. Interestingly, the interaction of the metals with the pyramidal side of BX3, where the lone pair in a hypothetical [BX3]2- should be located, is weaker than the interactions of metals with the inverted side, i. e., the middle of three halogen atoms. The origin of this stronger interaction can be explained by the formation of collective interactions between metals and halogen atoms as we explored via energy decomposition within the context of the theory of interacting quantum atoms, IQA.

Anti-electrostatic cation⋯π-hole and cation⋯lp-hole interactions are stabilized via collective interactions

Collective interactions are a novel type of bond between metals and AX3 fragments with an electropositive central atom, A, and electronegative X substituents. Here, using electrostatic potential maps and state-of-the-art bonding analysis tools we have shown that collective interactions are anti-electrostatic cation⋯π-Hole or cation⋯lp-Hole interactions.

Searching for Systems with Planar Hexacoordinate Carbons

Here, we present evidence that the D2h M2C50/2+ (M = Li-K, Be-Ca, Al-In, and Zn) species comprises planar hexacoordinate carbon (phC) structures that exhibit four covalent and two electrostatic interactions. These findings have been made possible using evolutionary methods for exploring the potential energy surface (AUTOMATON program) and the Interacting Quantum Atoms (IQA) methodology, which support the observed bonding interactions. It is worth noting, however, that these structures are not the global minimum. Nonetheless, incorporating two cyclopentadienyl anion ligands (Cp) into the CaC52+ system has enhanced the relative stability of the phC isomer. Moreover, cycloparaphenylene ([8]CPP) provides system protection and kinetic stability. These results indicate that using appropriate ligands presents a promising approach for expanding the chemistry of phC species.

Inter-Alkali-Metal Dative Bond in the MMN3 - (M=Alkali Metal) Cluster

Alkali metals are generally Lewis acids. On the contrary, Lewis basic character of alkali metals forming donor - acceptor complexes is a very rare phenomenon. In this contribution, I have theoretically designed an anionic cluster MMN₃ ^- (M=alkali metals) on the basis of experimentally known reagent, alkali salt of azide ion MN₃ , which shows unprecedented M:^- →M donor-acceptor interaction. To the best of author's knowledge, the characterization of such donor-acceptor interaction among alkali metals is unprecedented. Formation of the 2c-2e donor-acceptor bonds have been confirmed by quantum theory of atoms in molecules and electron localization function analyses. The calculated bond dissociation energies are significant suggesting their possible spectroscopic identification.

Can an alkalide act as a perfect Lewis base?

The Lewis basic character of alkali metals forming donor-acceptor complexes is a very rare phenomenon. No Lewis adduct with an alkalide as the Lewis basic centre has ever been reported. Herein, we theoretically designed EXH₂^- (E = Li, Na, K; X = Be, Mg, Ca) clusters which represent the first true example of Lewis adducts with alkalides as the two-electron donor basic sites. Our high level ab initio calculations reveal the formation of an unprecedented E:^- → XH₂ donor-acceptor interaction. Topological analysis within the realm of the electron localization function confirms this bonding scenario. The bonding scenario is exactly replicated in all the clusters, rendering support to our proposal. The calculated bond dissociation energies are significant, suggesting their possible spectroscopic identification.

Collective interactions among organometallics are exotic bonds hidden on lab shelves

Recent discovery of an unusual bond between Na and B in NaBH₃^- motivated us to look for potentially similar bonds, which remained unnoticed among systems isoelectronic with NaBH₃^-. Here, we report a novel family of collective interactions and a measure called exchange-correlation interaction collectivity index (ICI_XC; [Formula: see text]) to characterize the extent of collective versus pairwise bonding. Unlike conventional bonds in which ICI_XC remains close to one, in collective interactions ICI_XC may approach zero. We show that collective interactions are commonplace among widely used organometallics, as well as among boron and aluminum complexes with the general formula [M^a+AR₃]^b- (A: C, B or Al). In these species, the metal atom interacts more efficiently with the substituents (R) on the central atoms than the central atoms (A) upon forming efficient collective interactions. Furthermore, collective interactions were also found among fluorine atoms of XF_n systems (X: B or C). Some of organolithium and organomagnesium species have the lowest ICI_XC among the more than 100 studied systems revealing the fact that collective interactions are rather a rule than an exception among organometallic species.

Protonation of Borylated Carboxonium Derivative [2,6-B10H8O2CCH3]−: Theoretical and Experimental Investigation

The process of protonation of [2,6-B10H8O2CCH3]− was investigated both theoretically and experimentally. The most suitable conditions for protonation of the derivative [2,6-B10H8O2CCH3]− were found. The process of protonation was carried out in the presence of an excess of trifluoromethanesulfonic acid CF3SO3H at room temperature in dichloromethane solution. The structure of the resulting complex [2,6-B10H8O2CCH3*Hfac]0 was established using NMR data and the results of DFT calculations. An additional proton atom Hfac was found to be localized on one of the facets that was opposite the boron atom in a substituted position, and which bonded mainly with one apical boron atom. The main descriptors of the B-Hfac bond were established theoretically using QTAIM and NBO approaches. In addition, the mechanism of [2,6-B10H8O2CCH3]− protonation was investigated.

Planar Hypercoordinate Carbons in Alkali Metal Decorated CE3 2- and CE2 2- Dianions

After exploring the potential energy surfaces of M_m CE₂ ^p (E=S-Te, M=Li-Cs, m=2, 3 and p=m-2) and M_n CE₃ ^q (E=S-Te, M=Li-Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE₂ ^2- and Y-shaped CE₃ ^2- dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals.

Na⋯B bond in NaBH 3 - : An induced spin-polarized bond

The nature of the Na⋯B bond, in the recently synthesized NaBH 3 - adduct, is analyzed on the light of the Na^- propensity to polarize along the bond axis as a consequence of the electric field produced by the BH₃ fragment. The observed induced polarization has two consequences: (i) the energetic stabilization of the Na^- , and (ii) the split of its valence electrons into two opposite lobes along the bond axis. Additionally, an analysis of the electron localization is presented using the information content of the correlated conditional pair density that reveals a significant delocalization between one lobe of the polarized Na^- anion and the BH₃ fragment at the equilibrium distance. Our findings reported here complement previous works on this system.

A Critical Look at Linus Pauling's Influence on the Understanding of Chemical Bonding

The influence of Linus Pauling on the understanding of chemical bonding is critically examined. Pauling deserves credit for presenting a connection between the quantum theoretical description of chemical bonding and Gilbert Lewis's classical bonding model of localized electron pair bonds for a wide range of chemistry. Using the concept of resonance that he introduced, he was able to present a consistent description of chemical bonding for molecules, metals, and ionic crystals which was used by many chemists and subsequently found its way into chemistry textbooks. However, his one-sided restriction to the valence bond method and his rejection of the molecular orbital approach hindered further development of chemical bonding theory for a while and his close association of the heuristic Lewis binding model with the quantum chemical VB approach led to misleading ideas until today.