METAL CLUSTER

BY

DR. SHEERIN MASROOR

For

M.Sc Sem (II)

Clusters in Chemistry

• These are assembly of atoms or molecules

which are intermediate in size between a

molecule and a bulk solid.

• The can be made up of diverse in

stoichiometry. (Mingos and Wales 1990,

Introduction to Cluster Chemistry)

• Examples: Fullerene, Diborane.

Metal Cluster

• These are compounds that contain metal-metal bonds.

• A compound that contains a group of two or more atoms

where direct and substantial metal bonding is present.

(Huheey Inorganic Chemistry Huheey, JE,Harper and

Row, New York)

• Examples: Co2(CO)8, Rh2(OAc)4, Stryker’s

reagent(Osborn Complex).

Rhodium(II) acetateDicobalt octacarbonyl

hexameric copper hydride

ligatedwith triphenylphosphi

ne

Metal Clusters History• 12th century - Historically Metal cluster was calomel (Hg2Cl2).

• 1826 - First polyoxometallate discovered was ammonium

phosphomolybdate, containing the PMo12O3−

40 anion, discovered by

Berzelius.

• 1908 - Miolatti-Rosenheim explained about the structure of 12-

molybdophosphoric acid as a polyoxometalate cluster.

• 1928 - Linus Pauling suggested structure that accurately accounts for

structure of polyoxotungstate anion.

• 1934 - J. F. Keggin obtains first crystal structure of polyoxo-

metallate. Now known as Keggin stucture.

• 1953 - X-Ray Evidence for Metal-to-Metal Bonds in Cupric and

Chromous Acetate.

• 1964 - F. A. Cotton (Wilkinson Student) discovers Re2Cl8-2 which was

having metal-metal bond.

• Early 1960s - Cotton coins terms cluster when referring to chemical

entities that contain one or more metal-metal bond.

Preparation of Metal Clusters• Hydrothermal Synthesis

• Crystallization of substances from high temperature conditions.

• Generally reactions are carried in aqueous media which run in steel

pressure vessels in an autoclave.

• The first growth of a crystal was by Karl Emil with the synthesis of

quartz crystal.

• Robert Bunsen also what a major developer of the technique.

• Solution synthesis• The reactions are generally carried out in flasks, filtered, and product is

crystallized.

• Sometimes crystallization times takes around week or many.

• For Examples: clusters made by this method are polyoxometalates,

iron-oxo clusters, etc

• Solid-state synthesis• The reactions are generally taken out in alumina or silver boats at very

high temperatures.

• Usually reaction times are on the order of days to weeks.

• Products are typically characterized by powder x-ray diffraction analysis.

• Reductive methods under CO atmosphere• Reactions usually start with an air stable oxidized form with stoichiometric

metallic reductant.

• Some methods employ 1 atm of CO, while others require harsher pressures.

• Most methods for preparation require inert atmosphere or “filtering under

CO.”

• Products are typically characterized by elemental analysis and x-ray

diffraction.

Boranes

• A Borane is a chemical compound of boron and hydrogen.

• General formula of boranes is BxHy.

• The parent member BH3 is called borane, is found only in

the gaseous state, and dimerizes to form diborane, B2H6.

• The synthesis of the boranes, identification of their structure

and bonding principles and the development of their

chemistry began in 1910 with the work of Alfred Stock.

• Most of them are high reactive and they immediatly oxidize

on contact with air.

• So a chemical glass vacuum line (Stock-apparatus) is

needed for the synthesis.

Ores of Boron

• The principal ores of boron are borates such

as:

• Borax {Na2[B4O5(OH)4].8H2O}

• Colemanite {Ca2[B3O4(OH)3]2.2H2O}

Kernite {Na2[B4O5(OH)4].2H2O}

• The chemistry of the boranes was first studied systematically

by Alfred Stock and his research group beginning in 1912 and

continuing for almost 25 years.

• His initial work involved the reaction of acids with

magnesium boride, although better routes have been

developed since then.

• Because these compounds are exceedingly air-sensitive (they

instantaneously burn in air) he developed the glass vacuum

line and techniques for using it.

• Another giant in the field of boranes is W.N. Lipscomb of

Harvard Univ. whose work on the theory of bonding within

boranes (vide infra) led to his being awarded the Nobel Prize

in Chemistry in 1976.

• Also, H.C. Brown (Purdue) shared the 1979 Nobel prize for

synthetic work in organic chemistry utilizing boranes.

• Boranes are named as follows:

• a) The Latin prefixes mono-, di-, tri-, etc.

are used before "borane" to indicate the

number of boron atoms in the compound.

• b) Immediately following the "e" in

"borane" the number of hydrogen atoms is

placed in parentheses using Arabic

numerals.

• Example: B5H11 is pentaborane(11).

Bonding in Boranes

• The diborane molecule has 2 type of bonds.

• • 1. four terminal (2c-2e) B-H bonds.

• • 2. two bridged (3c-2e) B-H-B bonds.

Bonds in Boranes

• 1. Four terminal (2c-2e) B-H bonds.

Each of these bonds are formed by sharing of 2

electrons between boron and terminal

hydrogen atoms. It’s a normal 𝜎 covalent

bond.

2. two bridged (3c-2e) B-H-B bonds.

Each of these bond is formed by sharing of 2

electrons between 2 B and 1 H atoms .

It is also called as Banana bond.

C2H6B2H6

Boron is sp3 hybridised.

4 normal covalent bond

(2e-2c) (sp3-s)

2 banana bond (2e-3c)

(sp3-s-sp3)

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

Bond Angle and Bond Lengths

in Diborane

• The concept of three centre-two electron (3c-2e) B-H-B

bond proposed by Longuet Higgins was introduced.

• Covalent bonding theory, suggest molecular orbitals are

formed by linear combination of atomic orbitals.

• In the same way, if three atomic orbitals of same symmetry

and similar energy combine linearly, these will give rise to

three molecular orbitals.

Classification of Bonds in

Higher Boranes

• The B-H-B bridges includes in Bn

framework, while B-H are discarded.

• Some kind of bonds present in boranes are

as given:

Terminal B-H Bond

• This is normal 2e-2c covalent bond and is

generally shown as B-H.

Direct B-B Bond

• This is a 2e-2c covalent bond and is present

in between two boron atoms generally

shown as B-B.

Bridging or Open B-H-B Bond

• This is 2e-3c bond and is able to connect

two boron and one hydrogen atom.

Represented by B-H-B.

Synthesis of Boranes

• Diborane can be synthesized by an exchange reaction of a

boron halide with LiAlH4 or LiBH4 in ether, for example:

• 3 LiAlH4 +4BF3⟶ B2H6 + 3LiAlF4

• The reaction has to be done under vacuum or with exclusion of

air, because diborane burns in contact with air.

• Higher boranes are obtained by controlled pyrolysis of diborane

in the gas phase.

• For example:

• B2H6 (g)⟶2BH3 (g)

• B2H6(g)+BH3(g)⟶ B3H7(g)+H2(g)

• BH3(g)+B3H7(g)⟶ B4H10(g)