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Boron and hydrogen can form a series of hydrides, namely borane , which is an extremely important class of boron-containing compounds
.
Since the first case of borane in the early 20th century, more than 20 kinds of borane have been synthesized so far
1.
Preparation of Diborane
The most important boron hydride is diborane, and most of the other advanced boranes are made from diborane
.
Diborane can be prepared by various methods.
(1) Diborane is prepared by the reaction of dilute acid and metal boride
.
The long-term preparation of borane is realized by the action of Mg 3 B 2 and hydrochloric acid
.
(2) In the ether medium, LiAlH 4 reduces BCI 3 to prepare high-purity diborane
.
3LiAlH 4 +4BC1 3 =2B 2 H 6 +3AICl 3 +3LiCl
(3) Diborane can also be generated by reducing BC1 3 with hydrogen under discharge conditions
.
2BCl 3 +6H 2 =B 2 H 6 +6HCI
2.
The properties of ethyl ethane 7/8
(1) The stability of diborane is poor, and it decomposes at room temperature (in fact, polymerization reaction occurs) and produces a mixture of higher borane
.
2B 2 H 6 =B 4 H 10 +H 2
(2) Diborane has a strong reducing ability, spontaneously combusts in the air, and is easily oxidized by an oxidant
.
(3) Diborane is easily hydrolyzed, and an oxidation-reduction reaction occurs in essence (the oxidation number of B remains unchanged)
.
B 2 H 6 +6H 2 O=2H 3 BO 3 +6H 2
(4) As a Lewis acid, diborane can react with a variety of Lewis bases
.
For example, diborane can react with lithium hydride to form LiBH 4 , and the H -in LiH provides an electron pair
B 2 H 6 +2LiH=2LiBH 4
(5) Diborane can react with ammonia to generate B 3 N 3 H 6 at high temperature
.
3B 2 H 6 +6NH 3 = 2B 3 N 3 H 6 +12H 2
The molecular structure of B 3 N 3 H 6 is shown in Figure 13-9
.
Because the molecular structure of B 3 N 3 H 6 is similar to that of benzene, and it is an isoelectron with benzene, it is called inorganic benzene
.
The B and N in the molecule are arranged alternately, and both B and N adopt sp 2 equivalent hybridization
Figure 13-9 Schematic diagram of the structure of an inorganic benzene molecule
(6) Diborane reacts with ammonia gas at 873K to obtain boron nitride with a layered structure
.
nB.
H+2nNHs-2(BN)
3.
The structure of borane
Boron has 4 valence layer orbitals (1 2s, 3 2p), but only 3 valence electrons, so the formed borane is an "electron-deficient" compound
.
Therefore, compared with alkanes, there is a big difference in structure, and its structure is more diverse.
In addition to the formation of ordinary two-center two-electron bonds (such as BH and BB bonds), it also forms three-center bonds and multi-center bonds
.
There are three main types of borohydrides, B n H n 2- has a cage structure (closed triangular polyhedron), B n H n+4 has a nested structure (triangular polyhedron lacks a vertex), B n H n+6 It has a network structure (triangular polyhedron lacks two adjacent vertices)
.
In the borane structure, H is not regarded as the apex of the polyhedron
.
For example, B 6 H 6 2- has a regular octahedral structure, and B 5 H 9 has a tetragonal pyramid structure (the octahedron lacks a vertex)
.
Diborane B 2 H 6 is a dimer of BH 3 , and 2 bridging hydrogens and 2 terminal hydrogens around each B form a tetrahedral coordination
.
The planes of the 2 B and 4 terminal hydrogens in the B 2 H 6 molecule are perpendicular to the planes of the 2 grids B and 2 bridging hydrogens, that is, the 2 Bs and 4 terminal hydrogens are in the same plane, and the 2 bridging hydrogens are located on the plane respectively.
The upper and lower sides of the battery are shown in Figure 13-10
.
In the B 2 H 6 molecule, the B atom adopts sp3 hybridization.
Among the 4 hybrid orbitals, 2 single-electron orbitals and 2 1s orbitals with terminal H are used to form a σ bond; each B atom also has 2 heteroatoms.
Chemical orbital (only one electron), two bridging hydrogens each with one electron, two B atoms and two bridging hydrogens form two three-center two-electron hydrogen bridge bonds
.
Figure 13-10 The structure of diborane
There are mainly the following 5 bond types in various boranes
The boron-hydrogen bond and the boron-boron bond belong to the classic two-center two-electron
.
The hydrogen bridge bond is a new type of bond
.
The hydrogen bridge bond is formed by overlapping the 1s orbitals of H and the sp 3 hybrid orbitals of two Bs.
It is named because H is located in the middle of the two Bs
.
Among them, H provides one electron, one B provides one electron, and the other B only provides an empty orbital, so the hydrogen bridge bond is a three-center two-electron bond
.
The boron bridge bond is also a new bond
.
Different from the hydrogen bridge bond, a B replaces the bridge H and is located in the middle of the two Bs.
The boron bridge bond is also a three-center two-electron bond
.
The closed boron bond is also a three-center two-electron bond
.
Unlike the boron bridge bond, the three B atoms are equivalent to each other, and no B is located on the bridge
.
These five bond types exist in the B 10 H 14 (decaborane-14) molecule
.
The bond formation of the B 10 H 14 molecule is shown in Figure 13-11(a), and the spatial structure of the molecule is shown in Figure 13-11(b)
.
Each B in the molecule forms a boron-hydrogen bond with H.
Other bond types and bond numbers are as follows:
Figure 13-11 Schematic diagram of bonding and structure of B 10 H 14 molecule
