Isomeric hypho-boranes. Properties and molecular structures of the adducts B5H9·L (L = Ph2PCH2PPh2, Ph2PCH2CH2PPh2, or Me2NCH2CH2NMe2) and B4H8·L (L = Me2NCH2CH2NMe2)

Abstract

The air-stable adducts B₅H₉·L [L = bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), and NNN′N′-tetramethylethylenediamine (tmen)] and B₄H₈˙tmen are formed by the action of the ligand on the borane at room temperature; no cleavage of the B₅ group to form BH₃·L species occurs, even with an excess of ligand. The B₅H₉ complexes, which may be regarded as derivatives of the hypho-B₅H₁₁^2– ion, have been fully characterised by X-ray methods; for B₄H₈˙tmen an approximate structure is reported. The dppm and dppe structures both show the phosphorus atoms bridging the apical and basal atoms of a flattened pyramidal B₅ framework (with rearrangement of the hydrogen atoms). The tmen complexes differ markedly; each has the ligand chelating one of the originally basal boron atoms, which becomes singly bonded to the apical boron atom and is separated from the remaining basal atoms by typical non-bonded contact distances.