Wednesday, July 27, 2011

Summary notes of Transition Metals and their compounds

Part One (Part Two is also found on the sidebar)

* I found out that RJC notes on this chapter is good. You may want to grab hold of it, if possible. 

The notes found on this post are grossly inadequate for 2013 syllabus. So be forewarned. Students are advised to grasp the basics, then consult a university general chemistry or inorganic chemistry textbook.

** The toughest questions can be found from this chapter, which many teachers rush through due to insufficient time. Much reading is required.


Please note that the color spectrum and its related frequencies, wavelength, crystal field theory, Kstab are not discussed in these notes. You may want to study these concepts as this topic are highly important.

My notes are incomplete; I hope to improve on this post as this has turned out to be a popular post.

Chemistry of transition elements (These notes are modified from JC notes)There are about 50 transition elements and they have interesting chemical properties such as a variety of different oxidation states in their compounds, their extensive ability to form complexes, useful and often colourful compounds.

We shall only consider the first row transition elements.


A transition element is a d-block element that forms some compounds containing its ion with an incomplete d-subshell.


Strictly speaking, Sc and Zn are not considered to be transition elements.


You have to know the electronic configurations of the elements and their ions by looking at the periodic table.


 
The 3d block contains ten elements, because the 3d subshell contains five orbitals, each can accommodate 2 electrons of opposite spins. All these elements involve the filling of the 3d set of orbitals.


The 4s orbital is lower in energy than 3d. 4s orbital is filled before 3d orbital.


However, exceptions:

Cr [Ar] 3d5 4s1
Cu [Ar] 3d10 4s1
3d5 - half filled d orbital is preferred.
3d10 – full d-orbital

Why? Symmetrical 3d cloud of electrons that screens the nucleus more effectively than the other configurations.


Before electrons occupy 4s and 3d, 4s is energetically more stable than 3d.

When 3d orbitals are occupied by electrons, they repel the 4s electrons even further from the nucleus and up to a higher energy level than the 3d orbitals.

Thus, the energy level of the 4s orbital is higher than that of the 3d orbitals.


Hence in forming ions, 4s electrons are removed before the 3d electrons.


Properties


Across transition elements: trend in atomic radii.

Decrease slightly/ remains relatively constant
Reasons : Nuclear charge increases Due to increase in No. Of protons
Electrons are added to the inner 3d subshell.
Screening effect increases.
Effective nuclear charge increases Slightly.

Across transition elements: trend in ionic radii

Decreases slightly/remains relatively constant
Reason: Electrons are removed from the outermost shell
Screening effect inc. With inc. 3d electrons
Nuclear charge also increases.
Effective nuclear charge increases slightly.

Trend in 1st I.E.

Increases only slightly
Reason : 1st IE involves removal of a 4s electron
Nuclear charge inc. Across the d-block, inc. In 3d electrons shield the effect of increasing nuclear charge so that the 4s electron experiences only a small extra attraction.
Energy required to remove the outermost electrons inc. Very slightly.

Trend in m.p. and b.p.

Generally higher than s-block metals
Both 4s and 3d electrons are involved in metallic bonding
Mn has a relatively low mp and bp. Half-filled d-subshell, 3d electrons are less available for delocalization. Weaker metallic bonding.
Fe to Zn, mp and bp generally decreases as electrons being to pair up in the 3d orbitals. Less participation in metallic bonding.

Trend in densities : very high

Higher atomic masses
Small atomic radius, close packing of atoms, resulting in more no. Of atoms per unit volume.
Sc to Cu, inc. In density

Electrical conductivity : Very good conductor of heat and electricity.

Can use 4s and inner 3d electrons.

Chemical properties:

 

Oxidation states : can form often colored ions and compounds with the metal having different oxidation states.


eg. manganese in its compounds can exhibit a range of oxidation numbers ranging from +2 to +7.


The metals and their compounds often show catalytic properties.


eg. In the Contact Process (wiki this, this is out of syllabus but came out in exam before), vanadium (V) oxide is used as a catalyst to convert sulfur dioxide into sulfur trioxide.


They have a strong tendency to form complex ions.


They form often coloured ions or compounds.


Why do the various transition metal ions have variable oxidation states?


Reason : Close similarity in energy of the 4s and 3d electrons. Hence, once the 4s electrons are removed, some or all of the 3d electrons may be removed without requiring much more energy.


General trends in oxidation states:


In elemental states, elements assigned an oxidation number of zero.


No. Of available oxidation states of the elements increases from Sc to Mn.

There is a decrease in the no. of oxidation states exhibited by the elements Mn to Zn. Pairing of d-electrons occurs.


 

Lower oxidation states are usually found in ionic compounds.


The higher oxidation states tend to involve covalency (eg. TiO2, V2O5, Mn2O7) and oxo-anions (eg. MnO4-)

 

Redox reactions:


Generally, ions that have the transition metal in a high oxidation state tend to be good oxidising agents.


Early transition metals in low oxidation states tend to be good reducing agents. Cr2+, V2+.


Usually color change when transition metal ions undergo redox reactions.


You can use electrochemistry principles in determining Eθ cell and predicting whether a reaction is feasible.


Question : is there a redox reaction when V(NO3)2 reacts with KMnO4?

Transition Metals Part 2

Catalytic activity

Transition elements, both in the form of elements and in their compounds, are important catalysts in organic synthesis and in chemical industries.

Definition : A catalyst is a substance that increases the rate of a reaction by providing an alternative reaction pathway which has lower activation energy than the uncatalysed reaction.

The catalyst takes part in the reaction but does not undergo any permanent chemical change after the end of the reaction.

There are two types of catalysts:
Heterogeneous catalysts : they operate in a different physical state (phase) to the reactants.
Homogeneous catalysts : they operate in the same phase as the reactants.

Heterogeneous catalysis:
Haber Process:
Equation :


Catalyst : finely divided iron
Firstly, gaseous reactant molecules, nitrogen and hydrogen gas molecules, diffuse towards surface of iron catalyst.

The molecules adsorb onto surface of Fe catalyst.

Fe 3d partially filled orbitals can accept electron pairs from reactant molecules.

The adsorption process bring reactant molecules closer together and in the correct orientation.

NN and HH bonds are weakened.
Chemical reaction occurs, bonds break and new bonds form.

The catalysed pathway involves lower activation energy.

Faster reaction occurs.

After reaction, product molecules, ammonia molecules desorb and diffuse away from catalyst surface so that active sites are exposed for further reaction.

Energy profile diagram:


Note:
Forward and reverse reactions are speeded up by catalyst. Products are not formed more in quantity, but more quickly.
A catalyst provides a different mechanism for the reaction, with a new, lower activation energy pathway.
A catalyst does not alter the enthalpy change of reaction.
Other catalysts: contact process, hydrogenation of vegetable oils to make margarine by catalyst nickel.

Homogeneous catalysis:

The ability of transition metals to vary their oxidation state is the key factor in their efficiency as homogeneous catalysts.
Iron(III) ions as catalyst.

Complex formation:
Transition elements can form complex compounds.

High charge density of transition metal cations.

The cations have low lying vacant d orbitals that can be used to accommodate the lone pair of electrons from the ligands, resulting in dative bond formation.

s-block cations : low polarizing power.

Complexes:

Transition metal cations, the water molecules involved in hydration are held to the cation by stronger forces, leading to the formation of aqua Complexes – general formula [M(H2O)6]n+.
Ligands in complexes:
L: --> Mn+

Examples of ligands: chloride ion, hydroxide ion, cyanide ion, water, ammonia, carbon monoxide

Definition:
Ligands can be classified according to the number of dative bonds that it forms with the central metal ion.

Mondentate ligand: Those that can form only one co-ordinate bond with a central metal ion (or atom).

Polydentate ligand: Those that bond through electron pairs on more than one donor atom.

Polydentate ligands are sometimes called chelating agents because of their ability to hold a metal ion like a claw. (like a crab's claw holding onto its victim [the metal ion] tightly)

Chelating ligands form stable complex ion with the central metal ion as the pincer-like grip of these polydentate ligands are able to hold the cation more securely. Eg. EDTA4- (you need to know how this ion looks like; refer to your lecture notes  or chemistry textbook)

Complexes :
A complex is formed by a central ion ( or atom) dative-bonded by surrounding ions or molecules (known as ligands)
L --> M <-- L
A complex can be neutral, cationic or anionic.
In order to maintain charge neutrality in the compound, the complex ion is typically associated with other ion, called counter ions.
A typical transition metal complex compound: the complex is [Co(NH3)6]Cl3, the complex ion is [Co(NH3)6]3+, the six NH3 molecules bonded to the central Co3+ are ligands, and the three choride ions are counter ions.

You need to know how to name the complexes. Refer to your lecture notes but this is not highly important.
Complex and  electrolyte in solution.
Net charge:
Co-ordination number indicates the total number of dative bonds around the central atom or ion. The most common values are 2, 4 and 6.
Silver diamine compounds. [Ag(NH3)2]+ ions are used in carbonyl chemistry to oxidise aldehydes to carboxylate ions with the formation of a silver mirror or a silver precipitate.

IMPORTANT!
Why are transition metal complexes often coloured?

Transition metal compounds are frequently coloured. This is due to the absorption of light in the visible region of the electromagnetic spectrum.

Crystal field theory:
In a free transition metal atom, all five orbitals have the same energy.
As the ligands approach the metal ion, their electron pairs repel electrons in the five d-orbital, causing all five d-orbitals increase in energy.
The ligands approach directly toward the lobes of the d x2 – y2 and d z2 orbitals but between the lobes of the d xy, d yz and d xz orbitals.

Energy of an electron in either the d x2 – y2 and d z2 orbitals is higher because of electrostatic repulsion.
There is an energy gap between them. Delta E.
Colours – low wavelengths – blue violet, green
High wavelength – red, orange.
d-d transition : Jumping of an electron to an higher energy orbital.
For most transition metal complexes, the delta E corresponds to energy in the visible energy region (400 TO 700 nm) of spectrum.

Why are Zn2+ ions non-coloured?

Nature of ligands
Different ligands have different effects on the splitting of the d-orbitals. The larger the splitting of energy levels, the shorter the wavelengths absorbed and the longer wavelength light is thus observed.

You need to know the Common transition metal compounds and their colours. IMPORTANT!

Haemoglobin:
Haem --> O2 molecule , forms a dative bond with the iron to form a species called oxyhaemoglobin.
CO --> hame
To form carboxyhaemoglobin.

Review question:

Why do transition metal ions show a high tendency to form complexes?

Is the formation of a complex a reaction between a Lewis acid and a Lewis base?

Shapes of complexes: You can use VSEPR model to predict the shape of the complexes. Common shapes are linear, tetrahedral, square planar and octahedral.




(Not covered in these notes)
You need to know about complementary colors too and colors of transition metal complexes came out in A level exams in 2013.

You need to know strong field ligands and weak field ligands, factors affecting the color of transition metal complexes, ligand exchange and Kstab, effect of ligand exchange on Eo values (electrochemistry). For SPA, you need to know inorganic qualitative analysis.

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