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Electronic Spectra of Coordination Compounds, The presence of color in…
Electronic Spectra of Coordination Compounds
Transition state
Charge transfer spectra
Occur between metal & ligand bond
Types of the transition : MLCT , LMCT
MLCT
Also known as metal oxidation transition
Requirement for transition occur
1) Metal has high oxidation potential ( has tendency to lose electron) and has high density of electron.
2) Ligand has high reduction potential ( gain electron ) and has unoccupied orbital
Metal involved (has low oxidation number) : Mo (0), Fe(II), Ti(III), Ligand (strong field ligand) : CN, CO, Pyridine
LMCT
also known as metal reduction spectra
Requirement for transition occur
1) Metal has high reduction potential : Mn(VII), Cr(VI), Ti(IV) 2)Ligand has high donating electron: Cl- , I-, Br-, O2-,S2-
3) Ligand has higher electron density while metal has empty orbital
Usually observed in visible & ultraviolet regions (200-800 nm)
The spectra has high intensity
Ion pair spectra
exist when there is counter ion.
There is shoulder in the spectra due to the nitrate groups
d-d spectra
Caused by splitting of d-orbital of metal
Give very weak absorption and observed in visible region (400-700 nm)
Ligand spectra
transition that comes from the ligand
only for organic molecules complex. (ex: en, EDTA)
Majority observed in UV region (200-400 nm)
The absorption almost identical as in their free molecule
The number of peak depend on configuration of d-electron
Number of peak of dn and d10-n complexes are similar
d2 & d8 give 3 peaks, d4 & d6 give 1 peak, d3 & d7 give 3 peaks and d5 give no peak
Term state of Free Atom
Couplings
Spin-spin coupling (S)
coupling of s quantum numbers of all electrons in atom
S = (s1+s2), (s1+s2-1), (s1+s2-2) ... (s1-s2)
Orbit-orbit coupling (L)
coupling of l quantum numbers of all orbitals in atom
L = (l1+l2), (l1+l2-1),(l1+l2-2)... (l1-l2)
Term state: L=1 (P) , 2 (D), 3(F), 4(G), 5(H)
Multiplicity : 2S+1
Spin-orbit coupling (J)
coupling of s and l of electrons
J = (L+S), (L+S-1), (L+S-2)... (L-S)
If J has one value called singlet, two values (doublet), three values (triplet), four values (quartet)
Term symbol:
Term State for d-electron system
d1 system
only allow the highest l , L=2
d2 system
has 2 possible arrangements (microstates)
allowed term symbols are : 3F, 3P, 1G, 1D, 1S
The total number of microstates = Generation x multiplicity or The total number of microstates = (2L+1)(2S+1)
Ground state
Term that has lowest energy (most stable state) & have highest multiplicity
If have more than one terms that have similar highest multiplicity, term that has highest L chosen as ground state.
The presence of color in coordination complexes is due to electronic transition
Energy level of electron in atom
