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Higher Boranes

Boron forms a large number of hydries (in addition to dibroane). More than 25 neutral Boranes and a large number of borane anions have been prepared and characterized. We are only dealing with some very simple higher neutral Boranes. These are B4H10, B5H9, B5H11, B6H10 and B10H14. These can be classified into two categories based on their stoichiometry and structure.

1. Nido-Boranes : (BnHn+4)
                                                     B2H6,B5H9,B6H10 and B10H14

The word nido is derived from the Latin word meaning ‘nest’. So they have a nest like open structure.

2. Arachno-boranes : (BnHn+6)

The word orachno-means spider’s web. Their structure is open form both ends. In these compounds boron atoms occupy n contiguous corners of an (n + 2) cornered polyhedron.

Boranes are usually named by indicating the number of B atoms with a Latin prefix and the number of H atoms by an Arabic number in parentheses.

B2H6_____ _ diborane (6)
B4H10_______tetraborane (10)
B5H9___ _____pentaborane (9)
B5H11______ _pentaborane (11)
B6H10______ hexaborane (10)
B10H14_____  decaborane (14)

(1)    Tetraborane (10) (B4H10) is best prepared by keeping B2H6 under pressure at 250 for 10 days.

(2)    (i) pentaborane-9 (B5H9) is prepared by passing a 1:5 mixture of B2H6 and H2 at below one atmospheric pressure through a furnace maintained at 2500 for 3 seconds.

(ii) Alternatively, B5H9 can be prepared by pyrolysing B2H6 for 2.5 days in hot/cold reactor at 1800/-800.

(3)    Pentaborane-11 (B5H11) can be prepared in 70% yield by the reaction of B2H6 and B4H10 in a hot/cold reactor maintained at + 1200/ - 300C.

(4)    Decaborane-14 (B10H14) is prepared by paralysis of B2H6 at 100-2000C in the presence of catalytic amounts of Lewis bases such as demethylether.

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