Hydride, Nature of hydride, cluster compound and Magnetic property of block element.

Magnetic property.

Generally, any substances show the magnetic behaviour of 5 types.

1)Paramagnetic substances/paramagnetism:

When a substance is placed in magnetic field, then its shows the magnetic behaviour. A substance which is attracted by the magnetic field are called as paramagnetic substance and the magnetic field is called paramagnetism. The paramagnetism is due to the presence of a unpaired electron in an atom or ions or molecules.

Most of the transition elements and their compounds show the paramagnetism because they certain partially field electron shells. Hence the magnetic moment of the transition elements and their compounds is related to the no of unpaired electrons .

Magnetic moment =n(n+2)BM

Where n= no of unpaired electroms.

B.M =Bhor magneton , unit of magnetic `moment.

The paramagnetism first then decrease . The maximum paramagnetism is seen around the middle or the series., In the particular d-orbitals, there can be the maximum of 5 unpaired electrons in a d5 case and hence the maximum magnetic moment.

2)Diamagnetic substances/Diamagnetism:

The substances having no unpaired electrons show a net diamagnetism. Such magnetic substance is repelled by the magnetic field and are orient themselves right angles to the magnetic field and hence the magnetic moment is zero.

3)Ferromagnetic substance/ferromagnetism: Ferromagnetism may be regarded as the special case of paramagnetism in which the magnet moment of an atom become aligned and the substance are magnetised which behaves as the magnetic field. The metals like Fe, Co,Ni,etc behaves as a good ferromagnetic substance.

Important question

What information can be obtained by knowing the magnetic moments of the coordination compounds complex compound?

1) Zero magnetic moments of the substance represent that it is diamagnetic and greater than zero value of magnetic moment represent that the substance is paramagnetic.

2)No. of unpaired electrons present in the atomic orbital can be determined by knowing the magnetic moment of that substance.

3)Values of magnetic moment are of great important in distinguishing whether the complex is high spin. These values can also be to predict the geometry of complex compounds.

Cluster compounds or cage compounds

The gap of compounds in which the metal bonding between the two or more same or different metal atoms exist is called as cluster compounds or cage compounds.The tendency to form the metal metal-bonding is higher in 4d and 5d series elements whereas some carbon compounds with metal metal-bonding are also formed by 3d series elements.

Classification of cluster compounds

• On the basis of the metal atom.
• On the basis of nature of the metal atom.
• On the basis of nature of ligand.

1. Classification on the basis of a number of the metal atom.

a. High nuclearity cluster compound(HNCC).

Those group of cluster compound in which the metal-metal bonding between the 6 or more atoms occurs is called as high nuclearity cluster compound.example include [Rh₆(CO₁₆], [Co₆(Co)₁₄ etc.

b. Low nuclearity cluster compound(LNCC).

The cluster compound containing less than 6 metal atom bonded together with metal-metal bonding are called as low nuclearity cluster compound example as, as [Fe₂(CO)₃, [Ir₄(CO)₁₂ etc.

2. On the basis of the nature of the metal atom.

a.Homonuclear cluster compound.

The cluster compound containing metal-metal bonding between the two or more same metal atoms are called as homonuclear clear cluster compound example includes, [Rh₆(CO₁₆], [Co₆(Co)₁₄ as [Fe₂(CO)_3, etc.

b.Heteronuclear cluster compound.

The cluster compound containing metal-metal bonding between the two or more different metal atoms is called as heteronuclear cluster compound eg. [CO₂Ir₂(CO)₁₂.

3.Classification on the basis of nature of the ligand.

a.Carbonyl type of cluster compound.

The cluster compound in which the ligand are carbonyl group(CO) are called as carbonyl type of cluster compound example as, [Rh6(CO16], [Co₆(Co)₁₄ as [Fe₂(CO)₃, etc.

b.Halide type of cluster compound.

The cluster compound in which the ligand are halide group (X) are called as halide type cluster compound. Example as [MO₆Cl₈]⁴⁺, [Re₂Cl₈]^2-, etc.

Important question

How does the ionisation potential vary in 3d series of transition elements?

D block element have higher ionisation energy than those of s-block element (alkali and alkali earth metal) and lower than those of p-block elements. In 3d series the values of ionisation energy as we move from left to right in the series I,e Sc to Zn , although the increase is not quite regular,e the increased in the ionisation energy is quite gradual or slow. The reason for the gradual increased in given as below.

As we move from left to right I,e from Sc to Zn, the nuclear charge increase.The increase nuclear charge would attract the outer electron cloud with greater force and hence the ionisation energy are expected o increase at each step.However, as the electron is added to 3d subshell at each next element, the outer electrons are shielded more and more.The effect of increasing nuclear charge in opposite by the additional screening effect of the nuclear and consequently, the ionisation energies increase but quite slowly.

In the series, Zn has the highest valve of ionisation energy.This is due to the extra stability associated with their completely filled 3d and 4s orbitals.

Hydride.

The binary compound of hydrogen is called hydrides.eg,NaH, CaH₂ etc.

Nature of interstitial hydride.

Many d-block elements, lanthanides and actinides at elevated temperature absorb hydrogen into the interstitial space existing between the atoms of comprising the metallic lattice without changing the crystal structure of the metal. However, the elements in the middle of the d-block element eg( Mn, CO, Fe, Rh, Ru) do not form a hydride.The absence of hydrides in this part of the periodic table is called hydrogen gap.

Many interstitial hydrides are non-stoichiometric in nature.The absorbed hydrogen can be expelled from the metallic lattice simply, by heating the hydride and by increasing the gas pressure. Thus these hydrides are good reducing agent and act as an effective catalyst for hydrogenation, due to their ability to provide H-bond rather than H₂ molecule example if these include, T₁H₁₇₃, ZrH₁₉₂ etc.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.