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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

Subject Chemistry

Paper No and Title 11. Inorganic Chemistry –III (Metal π-Complexes and Metal Clusters)

Module No and Title Module 21. Bonding in higher boranes

Module Tag CHE_P11_M21

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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Bonding in Boranes 3.1 Bonding in diborane (B2H6). 3.2 Different bonds in boranes 4. Bonding in higher boranes (B4H10, B5H9, B6H10, B5H11, B10H14) 5. Summary

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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

1. Learning Outcomes

After studying this module, you shall be able to

• Learn about structures of boranes • Know the bonding in boranes • Understand different kinds of bonds in boranes

2. Introduction

In this module we will see how bridge bonding in diborane could be described simply by taking two banana-shaped three center-two electron bonds (3c-2e). An understanding of the overall distribution of valence electrons of diborane requires a molecular orbital treatment by taking into account the symmetry of whole molecule. The concept of three-center two electron bond can be applied to a wide variety of other electron deficient compounds. In boron chemistry, three types of 3c-2e bonds find considerable applications: B-H-B bridge bonds, closed three center BBB bonds and open three center BBB bonds.

3. Bonding in boranes

The bonding in boranes must logically be taken up with diborane for which the concept of three centre-two electron (3c-2e) B-H-B bond proposed by Longuet-Higgins was introduced. The ideas were then extensively developed by Lipscomb and his coworkers. In covalent bonding theory, molecular orbitals are formed by linear combination of atomic orbitals, e.g. two atomic orbitals of the same symmetry and similar energy combine to form one bonding and one anti-bonding orbital. In a similar manner, if three atomic orbitals of same symmetry and similar energy combine linearly, these would give rise to three molecular orbitals (the number of atomic orbitals which combine linearly is always equal to the number of molecular orbitals formed).

3.1 Bonding in diborane (B2H6)

In B2H6 we have two boron atoms, each with an available orbital having electronic configuration 1s22s22p1 with three valence electrons. The structure of diborane is of great interest. The diborane is an electron deficient compound i.e. there are not enough valency electrons to form the expected number of covalent bonds. The electron diffraction and spectral studies have suggested a hydrogen bridge structure in which one hydrogen atom forms a bridge between two boron atoms, while the terminal hydrogen atoms form normal covalent bonds with boron atoms. In B2H6 there are two terminal BH2 units bonded by two bridging hydrogen atoms. In each terminal BH2 unit there exist ordinary two center-two electron B-H bonds. The each boron atom is left with one electron after forming two bonds with the hydrogen atoms (2c-2e bond). Let us now consider the bridging arrangement, BHB. The bridging hydrogen atom has one electron and one useful orbital, namely its 1s orbital. Each boron atom also has an available orbital having an electron. We now have two three-center (BHB) bonding molecular orbitals

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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

containing four electrons of which two electrons are contributed by two boron atoms(after the formation of BH2 terminal bonds) and one from each hydrogen atom. These four electrons now occupy the two bonding molecular orbitals and the bridge bonding system is completed. An identical three center bonding molecular orbital will be formed in the other bridging system BHB. It is assumed that boron atom in B2H6 are sp3 hybridized and form simple two center-two electron bonds to their terminal hydrogens. The two terminal BH2 groups are present in one plane and the bridged B-H-B bonds are present in a plane perpendicular to the plane containing the two BH2 groups as shown in figure 1. Since two valence electrons of each boron atom are used up in BH2 bonding, the remaining valence electron of each boron atom is used in BHB bonding. Thus each boron atom contributes one valence electron towards the 3c-2e B-H-B bond. The bonding molecular orbitals involved in B2H6 and the structure of molecule are represented below in figures 1-2.

Figure.1 Molecular structure of diborane (B2H6)

(2002)

Figure.2 Qualitative picture of bonding in

diborane (B2H6) (2002)

As already mentioned, the B-H-B bridges are considered as part of Bn framework. The valence electrons of the two boron atoms can thus be accounted for as follows: Total number of valence electrons of 2 boron and 6 hydrogen atoms = 6+6=12 Electrons used up in the two bridges = 4 Electrons used up in four terminal B-H bonds = 8

3.2 Different bonds in boranes

The following types of bonds (table 1) are thus assumed to be involved in bonding in case of boranes. The electrons involved in bonding in the framework (skeletal structure) of Bn cluster in boranes are referred to as ‘framework electrons’. The B-H-B bonds are considered as part of the Bn framework whereas the B-H terminal bonds are not. Although, the framework electrons of Bn cluster are highly delocalized, i.e they belong to all the boron atoms of the framework, yet the localized 3c-2e and 2c-2e molecular orbital approach can explain the main features of the structures of boranes Each boron atom has three valence electrons of which one electron is used up by each boron atom of the framework in the normal 2c-2e terminal B-H bond and the remaining two electrons are contributed towards bonding in Bn framework.

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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

Table1. Different types of bond possibility in boranes. O denotes B and denotes H.

a). 3c-2e triply bridged B-B-B bond

One bonding electron originally belongs to

B atoms

b). 3c-2e triply bridged B-H-B bond

Both the bonding electrons originally belong to B atoms

c). 2c-2e normal B-B bond B B

One bonding electron originally belongs to

each B atom

d). 2c-2e normal B-H bond B H

The bonding electrons originally belong to

the B and the H atoms.

However, B atom of each terminal BH2 group contributes one electron to Bn framework. Thus overall bonding in boranes is codified as a four digit number, called ‘styx’ number. The first digit represents the number of 3c-2e, B-H-B bonds in the structure. The second, third and fourth digits represent the number of the 3c-2e B-B-B bonds, normal 2c-2e B-B bonds and the terminal BH2 groups respectively. The nature of the bonding in some of the boranes is discussed below. The styx number of each borane is also given in brackets.

4. Example of bonding in higher borane

4.2 Bonding in higher boranes (B4H10, B5H9, B6H10, B5H11, B10H14)

Bonding in B4H10 The bonding in B4H10 can be easily explained more easily by writing the plane projections of its structure as shown in figure 2.

Figure 2.Structure of tetraborane (B4H10) (4012)

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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

There are, evidently, four B-H-B bridges, no B-B-B triple bridge, one B-B bond and two terminal BH2 groups in B4H10. Hence, out of the 12 valence electrons of four boron atoms, 4 electrons are used in forming four B-H-B bridge bonds; 2 electrons are used in one B-B bond and the remaining 6 electrons are used in the six terminal B-H bonds. Total no. of valence electrons of four boron atoms = 4 x 3 =12 Valence electrons of ten hydrogen atoms = 10 Total no. of valence electrons of four boron and ten hydrogen atoms = 22 Electrons used in 3c-2e four BHB bridges = 4 x 2 = 8 Electrons used in 2c-2e one B-B bond = 2 Electrons used in six terminal B-H bonds = 6 x 2 = 12 Bonding in B5H9 The bonding in B5H9 can be explained conveniently by considering the plane projection of its structure as shown in figure 3.

Figure.3 Structure of pentaborane (B5H9)(4120) Out of the 15 valence electrons of five boron atoms, 4 electrons are used up in four B-H-B bridge bonds, 2 electrons in one B-B-B triple bridge bond, 4 electrons in the two B-B bonds and remaining five electrons are used in the terminal B-H bonds. Total no. of valence electrons of five boron atoms = 5 x 3 =15 Valence electrons of nine hydrogen atoms = 9 Total no. of valence electrons of five boron and nine hydrogen atoms = 24 Electrons used in 3c-2e four BHB bridges = 4 x 2 = 8 Electrons used in 3c-2e one BBB bridges = 1 x 2 = 2 Electrons used in 2c-2e two B-B bond = 4 Electrons used in five terminal B-H bonds = 5 x 2 =10 Bonding in B6H10 The bonding in B6H10 can be explained satisfactorily by considering the plane projection of its structure as shown in figure 4.

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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

Figure 4 Structure of hexaborane (B6H10) (4220) Out of the 18 valence electrons of six boron atoms, 4 electrons are used in four B-H-B bridge bonds, 4 electrons in two B-B-B triple bridge bonds, 4 electrons in two triple bridge bonds, 4 electrons in two B-B bonds and the remaining 6 electrons in six terminal B-H bonds. Total no. of valence electrons of six boron atoms = 6 x 3 =18 Valence electrons of ten hydrogen atoms = 10 Total no. of valence electrons of five boron and nine hydrogen atoms = 28 Electrons used in 3c-2e four BHB bridges = 4 x 2 = 8 Electrons used in 3c-2e two BBB bridges = 2x 2 = 4 Electrons used in 2c-2e two B-B bond = 2 x 2 =4 Electrons used in six terminal B-H bonds = 6 x 2 =12 Bonding in B5H11 The bonding in B5H11 can be explained satisfactorily by considering the plane projection of its structure as shown in figure 5.

Figure 5 Structure of pentaborane (B5H11) (3203) There are 15 valence electrons of five boron atoms in B5H11 out of which 3 electrons are used in three B-H-B bridges bonds, 4 electrons in two B-B-B triple bridge bonds and the remaining 8 electrons in the eight terminal B-H bonds.

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CHEMISTRY

11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

Total no. of valence electrons of five boron atoms = 5 x 3 =15 Valence electrons of eleven hydrogen atoms = 11 Total no. of valence electrons of five boron and nine hydrogen atoms = 26 Electrons used in 3c-2e three BHB bridges = 3 x 2 = 6 Electrons used in 3c-2e two BBB bridges = 2 x 2 = 4 Electrons used in eight terminal B-H bonds = 8 x 2 =16 Bonding in B10H14 In B10H14, there are four B-B-B triple bridge bonds and two B-B-B bent bridge bonds having structure. (fig.6)

Figure 6. Structure of decaborane(B10H14) (4620)

Hence, out of the 30 valence electrons of ten boron atoms, 12 electrons are used in the six B-B-B bridges bonds, 4 electrons are used in four B-H bridge bonds and 4 electrons are used in the two B-B bonds. The remaining 10 electrons are used in the terminal B-H Bonds. Total no. of valence electrons of ten boron atoms = 10 x 3 =30 Valence electrons of fourteen hydrogen atoms = 14 Total no. of valence electrons of ten boron and fourteen hydrogen atoms =30+ 14 = 44 Electrons used in 3c-2e four BHB bridges = 4 x 2 = 8 Electrons used in 3c-2e six BBB bridges = 6 x 2 = 12 Electrons used in 2c-2e two B-B bond = 4 Electrons used in ten terminal B-H bonds = 10 x 2 =20

Table2. Number and types of bonds present in some boranes

Boranes Bridging (3c-2e) Closed (3c-2e) Direct (2c-2e) Terminal (2c-2e) (valance shell

electrons) B-H-B bonds B-B-B bonds B-B bonds B-H bonds

Diborane, B2H6 (12) 2 (2˟ 2 = 4 e-) _____ _____ 4 (4 ˟ 2 = 8 e-)

Tetraborane, B4H10 (22) 4 (4˟ 2 = 8 e-) _____ 1 (1˟ 2 = 2 e-) 6 (6 ˟ 2 = 12 e-)

Pentaborane, B5H9 (24) 4 (4˟ 2 = 8 e-) 1 (1˟ 2 = 2 e-) 2 (2˟ 2 = 4e-) 5 (5˟ 2 = 10 e-)

Pentaborane, B5H11 (26) 3 (3˟ 2 = 6 e-) 2 (2˟ 2 = 4 e-) _____ 8 (8˟ 2 = 16 e-)

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11. Inorganic Chemistry –III: (Metal π- Complexes and Metal Clusters ) Module 21. : Bonding in higher boranes

Hexaborane, B6H10 (28) 4 (4˟ 2 = 8 e-) 2 (2˟ 2 = 4 e-) 2 (2˟ 2 = 4e-) 6 (6˟ 2 = 12 e-)

Decaborane, B10H14 (44) 4 (4˟ 2 = 8 e-) 6 (6˟ 2 = 12 e-) 2(2˟ 2 = 4e-) 10(10˟ 2 = 20 e-)

5. Summary

Ø It is assumed that boron atoms in B2H6 are sp3 hybridized and form simple two centre-two electron bonds to their terminal hydrogens.

Ø The two terminal BH2 groups are present in one plane and the bridged B-H-B bonds are present in a plane perpendicular to the plane containing the two BH2 groups.

Ø Two valence electrons of each boron atom are used up in BH2 bonding, the remaining

valence electron of each boron atom is used in BHB bonding. Thus each boron atom contributes one valence electron towards the 3c-2e B-H-B bond.

Ø The two electrons responsible for the formation of a three center-two electron bond B-H-

B will occupy the bonding molecular orbital.

Ø The number and types of bonds in B2H6, B4H10, B5H9, B6H10, B5H11 and B10H14 are summarized in table.2