Large Hyperconjugative and Inductive Effects on CCX Bond Angles (X = Main‐Group Element): The Crystal Structure of Triethylborane and Ab Initio Investigations

Abstract

Remarkable deviations from expected geometries are found in seemingly simple compounds. Several structure determinations of molecules with tricoordinate boron and the structural element CH,CH,B revealed abnormally large C-

Molecular Crystal Structure and Electron Density Distribution in the Crystal of Pentaethyl-1,5-dicarba-closo-pentaborane [C2B3(Et)5] at 120 K

“…26 The low molecular symmetry of II in the crystal is related with the nonregular orientation of Et groups at the boron atoms with respect to the equatorial B 3 -plane (see Figure 1) and II, respectively), and C-B-C (93.16(1) and 92.8(2)°) in both molecules are very similar, probably indicating the rigidity of the cage structure. It is interesting to note that the mean B-C(1,5) distance 1.571(5) Å in the cage is almost the same as the mean B-C(Et) bond length 1.577(2) Å, and these values are close to the mean B-C bond length in the structure of triethylborane (1.573(1) Å in accord with the X-ray data 32 ).…”

“…It was shown that similar effects are caused by hyperconjugation, that influences the ZCC bond angles in main element (Z) ethyl compounds. 32 Thus, for a boryl group (Z ) -BH 2 ) in accord with the MP2/6-31G* calculation the ZCC angle is equal to 105.3°if the empty p orbital of the boron lies in the C-C-B plane, because this conformation is favorable for hyperconjugation. If the empty p orbital of the B-atom is perpendicular to the C-C-B plane, the C-C-B angle is influenced indirectly, first via decreasing the H-C-H bond angle (electrons are withdrawn from CH 2 orbital with π-symmetry), and increasing the C-C-B bond angle as a consequence.…”

Alkyl Abstraction from a Trialkylyttrium Complex [YR3(thf)2] (R = CH2SiMe3) Using a Group‐13 Element Lewis Acid ER3 (E = B, Al, Ga, In) – Structural Characterisation of the Ion Pair [YR2(thf)4]+[GaR4]– and of ER3 (E = B, Al, Ga)

“…[17] The three B-CSi angles [117.04°on average, ranging from 114.27(14)°to 119.73(15)°] deviate from the ideal tetrahedral angle 109.47°a t the carbon, which was already observed in other tris(alkyl) boron compounds (cf. BEt 3 : B-C-C average: 118.9°, [17] or BtBu 3 : B-C-C 120.6(2)° [ 18] ). In these compounds, hyperconjugation was invoked to justify these abnormally large values.…”

Allylic boranes are chemist’s best friends: Reactivity, applications, new opportunities

“…Complexation with bases (R 3 B L) which is the first stage of the most organoborane reactions (e.g. with the C@O, C@N, C"C bond) leads to the elongation of the B-C bond(s) (!1.62 Å) [9,12] and consequently to its (their) weakness.…”

“…The B-C bond in triallylborane (1.580(2) Å, gas electronography) [8] is longer than in triethylborane (1.573(2) Å) [9]. The energy of the B-C(allylic) bond was estimated to be ca 68 kcal/mol [10] which is lower in comparison with 82-87 kcal/mol for trialkylborane [11].…”