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Offline Dolphinsiu

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Charge Transfer Band
« on: April 21, 2008, 02:15:24 PM »
What is charge transfer band? Is it related to d-d transition or d-p transition in tetrahedral or other? Why it is always observed in strong field ligand??

Offline Alpha-Omega

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Re: Charge Transfer Band
« Reply #1 on: April 21, 2008, 06:50:48 PM »
Since you are referring to Inorganic Chemistry I assume you mean the charge transfer that occurs in metal complexes. MLCT (metal to ligand charge transfer) and/or LMCT (ligand to metal charge transfer) which occur in transition metal complexes.

Transition metal complexes are highly colored. The colors exhibited by these complexes are due to electronic transitions by the absorption of light.

Most transitions related to metal complexes are either "d-d transitions" or "charge transfer bands." In a d-d transition, an electron in a d orbital on the metal is excited by a photon to another d orbital of higher energy.

A charge transfer band facilitates the promotion of an electron from a metal-based orbital into an empty ligand based orbital-MLCT(Metal to Ligand Charge Transfer).

Charge Transfer Transitions:

Some transition metal complexes with no d-electrons are colored.  This is because there can be electronic transitions in the visible region that do not involve d-electrons:

MnO4- : In this case, electrons in filled oxygen based orbitals are excited into empty d-orbitals. This type of Ligand to Metal Charge Transfer band gives rise to the intense purple color of permanganate

CrO42- : The intense yellow color observed is due to the LMCT band.

Metal to ligand charge transfer bands also occur in the visible regions for some complexes.  Charge transfer transitions are often much more probable than d-d transitions. Hence the intense color of MnO4-.

MLCT complexes experience a partial transfer of electrons from the metal to the ligand. Usually this occurs for metals with highly-filled d orbitals capable of donating electrons into the antibonding orbitals of the ligand. The non-bonding d orbitals must match the antibonding orbitals in terms of size, shape, and symmetry

Conversely, there can be excitation of an electron in a ligand-based orbital into an empty metal-based orbital LMCT (Ligand to Metal Charge Transfer). These transitions can be observed/monitored by electronic spectroscopy such as UV-Vis.

LMCT complexes experience a partial transfer of electrons from the ligand to the metal. This is common when the metal has a high oxidation state (e.g., MnO4- where Mn is in the +7 oxidation state and CrO42- where Cr is in the +6 oxidation state).

The d-d transitions that occur in simple compounds of high symmetry can be assigned using Tanabe-Sugano diagrams. These assignments can be confirmed using computational chemistry.

Studies of electronic spectra of metal complexes provide information about structure and bonding. Absorptions arise from transitions between electronic energy levels:

– Transitions between metal-centered orbitals possessing d-character = d-d transitions (MC) (weak intensity, Laporte-forbidden)

– Transitions between metal- and ligand-centered orbitals = metal-to  ligand (MLCT)
or ligand-to-metal charge transfer transitions (LMCT) (strong intensity)

-Absorption bands are usually broad due to vibrational and rotational sublevels

Mn(II) has a d5 high spin electron configuration. All of the d-orbitals are occupied with one electron each none of the possible (d-d) transitions is spin allowed, since for any transition the spin of the electron must be reversed (both higher energy eg orbitals contain already one electron, according to the Pauli principle the spin of the second electron must be reversed) Therefore: all possible transitions are very weak, and Mn(H2O)62+ is very pale in color.

Charge Transfer Spectra

The d-d absorption bands of transition metals involve redistribution of electrons that are localized on the metal.  There are also electronic transitions in which an electron moves from a ligand-based orbital to a metal based orbital, or vice versa these absorption bands are generally very intense.

MLCT:  Metal to ligand charge transfer band (electron moves from metal to ligand)

EXAMPLE:  [Cu(phen)2]+ (yellow complex)

LMCT:  Ligand metal charge transfer band (electron moves from ligand to metal)

EXAMPLE: [Fe(SCN)(H2O)5]2+ (red complex)

The colors of most transition metal complexes arise as a consequence of the ligand field splitting.

The ligand field splitting depends upon the metal, the oxidation state of the metal, and the ligand type.

Spectrochemical Series

One of the important aspects of CFT is that all ligands are not identical when it comes to a causing separation of the d-orbitals. For transition metal compounds, we are well aware of the multitude of colours available for a given metal ion when the ligands or stereochemistry are varied. In octahedral complexes, this can be considered a reflection of the energy difference between the higher dz2, dx2-y2 (eg subset) and the dxy, dyz, dxz (t2g subset).

It has been established that the ability of ligands to cause a large splitting of the energy between the orbitals is essentially independent of the metal ion and the SPECTROCHEMICAL SERIES is a list of ligands ranked in order of their ability to cause large orbital separations.

• Effect of ligand is given by the spectrochemical series (order of ligand field strength with increasing delta q)

I- < Br- < S2- < SCN- < Cl- < NO3- < F- < OH- < C2O42- < H2O< NCS- < CH3CN < NH3 < en < bipy < phen < NO2- < PPh3 <CN- < CO

• High oxidation state favors large Δ (order of metals with increasing delta q):

Mn2+ < Ni2+ < Co2+ < Fe2+ < V2+ < Fe3+ < Co3+ < Mn4+ <Mo3+ < Rh3+ < Ru3+ < Pd4+ < Ir3+ < Pt4+

When metal ions that have between 4 and 7 electrons in the d orbitals form octahedral compounds, two possible electron allocations can occur. These are referred to as either weak field - strong field or high spin - low spin configurations.

One Explanation

Crystal Field Theory is based on the idea that a purely electrostatic interaction exists between the central metal ion and the ligands. This suggests that the stability of the complexes should be related to the ionic potential; that is, the charge to radius ratio.

In the Irving-Williams series, the trend is based on high-spin M(II) ions, so what needs to be considered is how the ionic radii vary across the d-block.

For high-spin octahedral complexes it is essential to consider the effect of the removal of the degeneracy of the d-orbitals by the crystal field. Here the d-electrons will initially add to the lower t2g orbitals before filling the eg orbitals since for octahedral 8 complexes, the t2g subset are directed in between the incoming ligands while the eg subset are directed towards the incoming ligands and cause maximum repulsion.

Selection Rules

For allowed transitions:

delta S = 0 The Spin Rule
delta l= +/- 1 The Orbital Rule (Laporte)

The first rule says that allowed transitions involve the promotion of electrons without a change in their spin.

The second rule says that transitions within a given set of p or d orbitals (i.e. those which only involve a redistribution of electrons within a given subshell) are forbiddenif the molecule has a centre of symmetry.

Relaxation of the Rules can occur through:

a) Spin-Orbit coupling - this gives rise to weak spin forbidden bands

b) Vibronic coupling - an octahedral complex may have allowed vibrations where the
molecule is asymmetric. Absorption of light at that moment is then possible.

c) pi-acceptor and pi-donor ligands can mix with the d-orbitals so transitions are no
longer purely d-d.

Types of transition

1) Charge transfer, either ligand to metal or metal to ligand often extremely intense and found in the UV but may have a tail into the visible.

2) d-d can occur in both UV and visible, weak transitions.

I have attached 2 Word documents:  CFT and LFT



« Last Edit: April 27, 2008, 08:32:24 AM by Alpha-Omega »

Offline Dolphinsiu

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Re: Charge Transfer Band
« Reply #2 on: April 23, 2008, 07:09:43 AM »
Thanks for your detailed explaination. I am sorry to tell you that I cannot download the attached document.

Offline Alpha-Omega

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Re: Charge Transfer Band
« Reply #3 on: April 23, 2008, 12:17:42 PM »
OK that is wierd....I am going to have to retype it....I can't open it either...I will get it back up there...seems to be a problem uploading word.doc documents...and it is telling me my copy does not exist...I will retype it an repost it

Offline Borek

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Re: Charge Transfer Band
« Reply #4 on: April 23, 2008, 12:20:55 PM »
Uploading word .doc documents is not a good idea - they are not considered safe. If you can export it as PDF that's much better.
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Offline Alpha-Omega

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Re: Charge Transfer Band
« Reply #5 on: April 23, 2008, 01:05:33 PM »
What do you mean export it?  Ok do you mean write in in word and then save in pdf format.  I typed that document myself and now for some reason it has not only not uploaded...but the file I saved I can't open...so something went very wrong.

If I do all tht work again I want to make sure it is saved and it uploads.  I do not have pdf writer....or maybe I do...not sure about that....and I do not see an export option in Word.

Well, no worries...I will get it uploaded one way or another... ;)

Both documents are attached and I am able to download them...If I fougure out the pdf thing I will try using it.

Problem solved...if you are saying word documents are unstable...I have a creator and converter from CNET...so I can do these in pdf format or just convert the word docs to pdfs....we will see if it makes a difference... ;)

Friend of mine at Purdue was able to steer me in the right direction.... ;D
« Last Edit: April 24, 2008, 04:03:02 AM by Alpha-Omega »

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