Analysis of Why Boron Avoids sp² Hybridization and Classical Structures in the B_nH_n+2 Series

Abstract

We performed global minimum searches for the B(n) H(n+2) (n=2-5) series and found that classical structures composed of 2c-2e B-H and B-B bonds become progressively less stable along the series. Relative energies increase from 2.9 kcal mol(-1) in B(2) H(4) to 62.3 kcal mol(-1) in B(5) H(7). We believe this occurs because boron atoms in the studied molecules are trying to avoid sp(2) hybridization and trigonal structure at the boron atoms, as in that case one 2p-AO is empty, which is highly unfavorable. This affinity of boron to have some electron density on all 2p-AOs and avoiding having one 2p-AO empty is a main reason why classical structures are not the most stable configurations and why multicenter bonding is so important for the studied boron-hydride clusters as well as for pure boron clusters and boron compounds in general.

Formation and Reactivity of Electron‐Precise B−B Single and Multiple Bonds

“…Moreover,t he electron-deficient nature of three-coordinate boron results in kinetic instability.U nless stabilized electronically-using pbasic (e.g. [11] For n ! [11] For n !…”

“…[12] Thef irst synthesis of an electron-precise diborane(4) (performed in 1925) was that of B 2 Cl 4 from BCl 3 by arc discharge between aZnanode and an Al cathode under N 2 at low temperature (Table 1). [15] Above 0 8 8C, B 2 Cl 4 was found to decompose to B 4 Cl 4, which, rather than ac lassical cyclotetraborane structure,a dopts am ore stable non-classical tetrahedrane structure in the solid state, [16] though 11 BNMR data and calculations suggest the existence of an electronprecise cyclo-tetrachlorotetraborane isomer in solution. [14] Unlike B 2 H 4 , diboron tetrahalides were shown to display aclassical planar D 2h alkene-like structure.…”

Homocatenation of Aluminum: Alkane‐like Structures of Li2Al2H6 and Li3Al3H8

“…[10] Earlier, the isoelectronic equivalence of carbon by B À has been shown for many species by Jemmis and Jayasree [11a] and Alexandrova et al [11b] Osorio et al have explored a variety of boron homocatenated compounds in the B n H n + 2 series, [12] which is supposed to be analogous to those of C n H 2n + 2 . Very recently, it was shown by Poater et al that classical organic polycyclic aromatic hydrocarbons (PAH) and closo-borohydride clusters share a common origin regulated by the number of valence electrons in an electronic confined space.…”

Einfach‐, Doppel‐, Dreifachbindungen und Ketten: Knüpfung elektronenpräziser B‐B‐Bindungen

“…B. Dialkylamino oder Alkoxy) am Boratom z. T. verhindert werden. Die Bedeutung dieses Konzepts der teilweisen Besetzung leerer Atomorbitale wurde vor kurzem von Tiznado und Boldyrev demonstriert, [2] deren rechnerische Prüfung der Geometrie von Boranen der Formel B n H n+2 (n = 2-5) ergab, dass Bor sp 2 -Hybridisierung vermeidet und dass die Bevorzugung nicht-klassischer Molekül-strukturen mit der Kettenlänge zunimmt. Es überrascht daher nicht, dass die große Mehrheit der längerkettigen Borane stark p-basische Aminosubstituenten aufweist, die diese Orbitale teilweise füllen und so den inheränten Elektronenmangel des Boratoms ausgleichen kçnnen.…”