COLOR OF THE TRANSITION ELEMENTS. Presenter: MS SAMIA SAADIA (Vice Principal) Subject: CHEMISTRY Class: XII. Defence Authority College for Women - Phase VIII. Colour inTansition Metal Complexes Application of Crystal Field Theory. CONTENTS.
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Presenter: MS SAMIA SAADIA (Vice Principal)
Defence Authority College for Women - Phase VIII
ColourinTansition Metal Complexes
Application of Crystal Field Theory
Transition elements are known by this name because they show their properties which are transitional between highly reactive and strong electropositive elements of s-block which form ionic bonds and highly electronegative elements of p-block elements which form largely covalent compounds.
B-group elements are transition elements
Transition elements consists of following :-
d- block elements
f- block elements
d- block elements are placed in the middle (in between the s & p- block elements) & f- block elements are placed at the bottom of the periodic table separately.
f- block elements
Complex or Coordination Compounds:
Metal atom or ion surrounded by oppositely charged ions or neutral molecules.
Polyatomic ion or molecule in which number of ions or molecules are bonded to a transition metal ion or atom through coordination bonds.
Tendency of Transition metals to form complex compounds:
Due to small size of the metal atoms or their cations, have a high charge density on them.
Makes the atom or cation to attract(accept) the lone pair of electrons from the ligands.
Have vacant d-orbitals [(n-1)d orbitals] to accommodate the lone pairs of electrons donated by ligands to form coordinate bonds (L→M).
When one or more neutral molecules or negatively charged ions become attached to central atom by coordination bond.
It is charged positively or negatively.
Ligand or Coordination group:
An ion or neutral molecules surrounding a centre atom
A part of complex compound which is directly attached with the central transition atom by coordination bonds.
Transition metals accept electrons from negatively charged
Ions and acts as Lewis acid or electrophile.
Legands are electron donors and are called Lewis Base or Nucleophile.
Coordination Number “n”:
The number of ligands attached to a centre atom or the number of coordination bonds formed with in a single central atom in a complex compounds. It is usaully fixed for a metal.
Cl <F <H2O <NH3 <en < NO2 (N-bonded) <CN
CRYSTAL FIELD THEORY
FIGURE : CRYSTAL FIELD THEORY
Splitting of Outer
D-orbital in octahedral field
Metal Ion Plus Coordinated Ligands
Central transition atom in complex ion contain partially filled
The color is due to the bonding between transition metal and ligand i.e electrostatic and is most successfully explained by Crystal Field Theory
The electrostatic field splits the five degenerated d- orbitals into two sets of energy levels namely t2g (triply degenerate) & engerade or eg (doubly degenerate)
T2g includes orbitals lie in between the axis (non-axial)
eg includes orbitals lie on the axis (axial)
This splitting causes unpaired electrons to transit triply degenerate from one set to another
The ∆E0 i.e. energy difference is too small and equivalent to the wavelength that electron jump from an orbital to the other
A high energy pair, dx2-y2 and dz2 designated as “eg” (Doubly degenerate)
A lower energy trio, dxy, dyz and dzx designated as “t2g” (Triply degenerate)
Color of the Transition Element
eg Cu1+(White), Cu2+(Blue)
eg [Zn(NH3)4]2+(White), [Fe(CN)]63-(Violet)
eg [Cu(H2O)6]2+(Blue), [Cu(NH3)2]2+(Deep Blue)
eg [Ti(H2O)6]3+ absorbs blue green color at 4900A-5000A & transmits Red-Purple color but [Ti(F)6]3- absorbs yellow color at 5750A- 5900A& transmits violet color.
Reason: changes weaker ligand smaller crystal field splitting at range of longer wave length.
Examples: A comparison of the visible absorption maxima for a number of cobalt (III) complexes shows the effects of ligands on the d-orbital band gap.
VISIBLE REGION OF THE SPECTRUM:
This excitation involves absorption of light (photons) of certain wave length in the region of visible light(3800A0-7800A0) and release of energy is observed in the form of colors ‘VIBGYOR’
This is called d-d transition which is responsible for the color
If we add the colors on opposite sides of the wheel together, white light is obtained.
We only detect colors when one or more of the wavelengths in the visible spectrum have been absorbed, and thus removed, by interaction with some chemical species.
When the wavelengths of one or more colors is absorbed, it is the colors on the opposite side of the color wheel that are transmitted.
a) Sample absorbs all , black color perceived
Sample does not absorb any color of light. White light is perceived
Grass and leaves appear green because chlorophyll absorbs wavelengths in the red and blue portion of the visible spectrum. The wavelengths in between (green) are transmitted.
Examples of Transition Metal Complexes:
The absorption band corresponds to the energy required to excite an electron from the t2g level to the eg level.
The energy possessed by a light wave is inversely proportional to its wavelength.
The Cu(II) solution transmits relatively high energy waves and absorbs the low energy wavelengths.
This indicates that the band gap between the two levels is relatively small for this ion in aqueous solution.Excitation of d electron in Cu(II) Complex
Anhydrous Cobalt Compounds:
They absorbs light for the excitation of their d-electrons.
When red color is absorbed from light, then the transmitted light consists of wavelengths corresponding to other colors of white light in which the blue color predominates and the cobalt compounds thus appear blue green.
Hydrated Cobaltic Compounds: absorbs light of a different wavelength blue green and therefore appears red in color.
Co, [Co(NH3)6]2+ appears blue because the incoming ligand interacts with the
d-orbital of transition metal ion.
Thus interaction results in the splitting of the energies of
It causes the unpaired electrons to transit from one set to
This excitation involves absorption & release of energy is
observed in the form of blue color in the visible region of spectrum.
Physical state:Anhydrous and hydrated species of a metal atom or ion absorb at different wavelengths that is why exhibit different colors.