Metal Cluster Higher Boranes

Ankita Verma semester 1

1. Samrat Prithviraj Chauhan Government College Ajmer 2021-22 Submitted By Ankita Verma M.Sc. Chemistry Semester 1st Department of chemistry METAL CLUSTER HIGHER BORANES

2. CONTENT 1 ) INTRODUCTION 2) TYPES OF BOND FOUND IN HIGHER BORANES 3)STRUCTURE AND BONDING IN HIGHER BORANE 4)CLASSIFICATION OF HIGHER BORANES 5) SYNTHESIS

3. INTRODUCTION The hydrides of boron are known as boranes. Boranes were firstly systematically synthesized and characterized during the period of 1910 to 1940 by the G erman C hemist Alferd Stalk . Stalk and his co-workers prepared a higher boranes by there This is follow :- Mg 3 B 2 + 6HCL -------> B 2 H 6 + 3MgCl 2   Boranes are all colourless and diamagnetic . They are reactive compounds and some pyrophoric . Boranes are generally represent by two general formula : B n H n+4 B n H n+6

4. B n H n+4 : These boranes are quite stable and have high melting point . General value of n=2,5,6,8,10………… 2. B n H n+6 : These boranes are unstable and heave low melting point. General value of n=4,5,6,9,10……. According to IUPAC system of nomenclature the boron atom in boranes indicated by prefix-di, tri, tetra, lena etc . Followed by no. which indicates the no Of hydrogen atom . Example -B 6 H 10 ( Hexaborane-10)

5. TYPESOFBONDFOUNDINHIGHERBORANES Higher boranes may contain few or all of the following types of bonds : Terminal (2c- -2e ) B-H bond: This is a normal covalent bond formed by overlapping of singly filled sp 3 hybrid orbital of boron atom and singly filled 1s – orbital of hydrogen atom . Direct (2c- -2e ) B—B bond: This is a normal covalent bond Formed by overlapping of singly filled sp 3 hybrid of two boron atoms . Bridging or open (3c- -2e ) B-H-B bond: This type of bond is formed by overlapping of two sp 3 hybrid orbitals of two boron atoms (one singly filled and other empty) and 1s orbital of hydrogen atom . Triply bridge or closed (3c- -2e ) B-B-B bond: This type of bond is formed by overlapping of three sp 3 hybrid orbital of three boron atoms. Out of these one sp3 hybrid orbitals empty. Thus each boron atom contributes 2/3 electrons for the formation of this bond . This type of bond may be represented as below:

6. STRUCTURE AND BONDING IN SOME HIGHER BORANES Structure of Diborance B2H6– Diboranes – 6 The diborane is an electron deficient compound i .e. there are not enough valence electrons to form the expected number of covalent bonds . In the structure of hydrogen atom form a bridge between two boron atoms while the terminal hydrogen atom form normal covalent bonds w ith boron atoms.

7. In this structure four hydrogen atom are known as terminal hydrogen's (Ht) and two other hydrogen atoms are known as bridging hydrogen's (Hb). The two boron atom and four terminal hydrogen atom lie on the same plane while two bridging hydrogen atom lie on a plane perpendicular to this plane. Types of bond in diborane structure Terminal (2c-2e) B-H bond = 4, B1-Ht, B1-Ht, B2Ht, B2-Ht . Bridging(3c-2e)B-H-B bond = 2, B1-Hb-B2, B1-Hb-B2.

8. B 4 H 10 :-(Tetraborane-10) In this molecule four B-Atoms form slightly distorted Octahedral geometry. Bridging (3c-2e) B-H-B bonds= 4, ( B¹-H-B³,B³-H-B²,B²-H-B⁴,B⁴-H-B¹ ). Direct (2c-2e) B-B bond=1, ( B¹-B² ). Terminal (2c-2e) B-H bond=6, ( B¹-H,B²-H,B³-H,B³-H,,B⁴-H,B⁴-H ).

9. B 5 H 9 =pentaborane-9 In this molecule five B –atoms are situated at five corners of a square pyramidal . Four B-atoms (B¹,B²,B³andB ⁴ ) are at the base and fifth B-atom (B ⁵) is At the apex of the pyramid.

10. Types of bond: Terminal (2c-2e) B-H bond=5, (B¹-H,B²-H,B³-H,B⁴-H,B⁵-H) Direct (2c-2e) B-B bond=2,(B¹-B⁵,B⁴-B⁵) Closed (3c-2e) B-B-B bond=1(B²-B⁵-B³) Bridging(3c-2e)B-H-B bond=4 ,(B¹-H-B²,B²-H-B³,B³-H-B⁴,B⁴-H-B

11. CLASSIFICATION OF HIGHER BORANES Boranes are classified into three classes on the basis of their skeletal structures. For this we should know the number of electrons present in multi centred bonding orbitals of the skeletal structure. Although the concept of localised 3c-2e and 2c-2e molecular orbitals, satisfactorily explain the bonding in the simple molecules but it is not of or complicated molecules (boranes ). Therefore a set of empirical rules are given which are called Wade's rules or polyhedral skeletal electron pair theory . These rule started as follow :- Wade's Rule:- In these rules it is assumed that- * Each BH unit contributes two electrons to skeletal bonding . * The negative charge is used up in skeletal bonding . * Each additional H atom in the molecule contribute one electron to the skeletal bonding .

12. On the basis of the above empirical rules the boranes are classified into following three types. Closo Boranes: These have completely closed triangular polyhedral structure in which all the vertices are occupied by boron atoms. If there are n boron atoms in the skeletal structure (or cage or frame work) of [B n H n ]² anion the number of electrons will be 2(n-1) i.e. 2n+2 then it is called closo Boranes . For example [B n H n ]² ion requires 26 electrons to fill up all the bonding molecular orbitals and correspond to 13 (n+1, n=13) electron pair and expected for an isohedron.

13. 2. Nido Boranes : If were move one boron atom from a vertex of a closo structure a nest or cup-like structure is obtained. Such as structures are known as Nido (Latin= nest) structure to satisfy the valencies of the corresponding boron atoms, extra hydrogen atoms are added. Therefore the Nido structure obey the frame work electron formula 2n+4. For Example : In the case of B5H9 the rear five B-H bonds which contributes two electrons each and four extra hydrogen atoms will contributes four electrons for a total of 14 (2n+4, n=5). These four H-atoms form bridges across the open edges of the nest. These corresponds to 7(n+2) electron pairs and the geometry of the molecule will be derived from an octahedron (n+1vertics). Thus we can explain the square pyramid Nido structure is derived from the closo octahedron.

14. 3 . Arachno Boranes: If we remove two vertex boron atoms of a closo structure or one vertex boron atom of an ido structure then we get arachno (Greek= spider's web) structure. Thus arachno structure has (n+3) electron pair in i.e. ( 2n+6 ) electrons. For example: Arachno pentaborane -11 [arachno B5H11] contains (n+3) i.e. ( 5+3 ) electron pair or 16 electrons. In other words 2×5+6=16 electrons in the multi centre bonding orbitals of their Skeletal structure.

15. SYNTHESIS Although the simplest boranes eg.B2H6 are spontaneously flammable in air burning with a characteristic green flame and very reactive toward solvents containing replaceable protons, reactivity generally decreases with increasing molecule weight. Some of the higher molecular weight polyhedral anions, such as B10BH10 and B12H12 are remarkably stable in air, water and heat. Arachno Boranes are generally more reactive and less thermally stable than nido boranes, which in turn are more reactive and less thermally stable than closo Boranes . Diboranes is more easily synthesized in high yield by reaction of iodine I² with sodium borohydride ( NaBH4 ) or lithium aluminium hydride (LiAlH4) in diglyme as a solvent.

16. REFERENCE ADVANCED INORGANIC CHEMISTRY BY: PRAGATI PRAKASHAN KIMTILAL

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