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-
Eher die Ausnahme als die Regel sind ideale Tetraederwinkel in Organoelementverbindungen. Die Abweichungen können beträchtlich sein, wie die Aufweitung des C‐C‐B‐Winkels in Triethylboran auf fast 120° (Strukturbild rechts) zeigt. Dies läßt sich mit einer hyperkonjugativen Wechselwirkung des leeren Bor‐p‐Orbitals mit den bindenden Orbitalen der α‐C‐H‐Bindungen erklären, was durch ab‐initio‐Berechnungen und Strukturbestimmungen anderer Verbindungen mit dem Strukturelement Et‐X (X = Hauptgruppenelement) gestützt wird.
ChemInform Abstract The rigid molecule (I) reacts with the sterically crowded pyrazoles (II) to give the monomeric species (III). The structure analysis of (IIIc) (space group P21/n, Z=4) together with the 13C NMR spectra indicate that the B atom is only bonded to one N atom of the pyrazolyl unit.
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