photo chemistry of ligand in coordination compound

Addis Ababa UniversityDepartment of Chemistry

PhD Program

Photochemistry of Ligand Field Transition

presenter: Masresha Amare

Adviser: Prof V.J .T Raju Dr. Yonas Chebude

Introduction

What is Photochemistry about?concerned with the changes in chemical and

physical behaviour of molecules following absorption of one (or more) photons

Primarily consider absorption of visible/UV although IR absorption may also change chemical behaviour

Mainly concerned with electronic excitation

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Electronic excitation change of molecular orbital occupancyincreased energychange of bonding characteristics and possibly geometrychange of charge distributionpossible changes of resultant electron spin, orbital symmetry

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AB*

Fates of photoexcited speciesPhysical quenching

AB

BAIsomerization

A + BDissociation

AB+ + e-

Ionization

LuminescenceAB + h

AB + CD‡

Intermolecular energy transfer

AB†

Intramolecular energy transfer (radiationless

transition)

AB + E or ABEDirect reaction

AB+ + E- or AB- + E+

Charge transfer

+ M

+ CD + E

Electronic Transitions

Three types of electronic transitions may be distinguishedtransitions between MOs mainly localized on the central metal. the metal d orbitals (e.g., the t2g(π) and eg MOs

d–d transitions or ligand-field transitions

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Octahedral complexes Cr(III)ground state of these complexes,4A2g, belongs to the configuration t2g

3.

Spin allowed 4A2g(t2g3) 4T2g(t2g

2eg1) and 4A2g 4T1g correspond to the promotion of

an electron from the t2g to eg orbitals.

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Effect of T2g eg

(1) an increase in the metal–ligand repulsionsome lengthening of metal-ligand distances breaking of a metal-ligand bond rearrangement of the molecule toward a more-stable structure (isomerization

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(2) a decrease of electron density in some directions between the ligands Facilitate a nucleophilic attack on the central metal by solvent molecules or other ligands present in the solution (substitution reaction)

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Transitions between MOs mainly localized on the ligands and MOs mainly localized on the central metal. This transition are called CT or optical electron transferLMCT or MLCT, transitions can occur.

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Transitions between MOs mainly localized on the ligands.only involve ligand orbitals which are almost unaffected by coordination to the metal Transitions of such a type are called ‘ligand-centered’ (LC) or ‘intra-ligand transitions.’

Absorption Bands

Absorption Bands

MC (Ligand-Field) Bands Consider a d1 octahedral complex such as

[Ti (H2O)6]2+.

all the orbitals lying below the t2g(π*) orbitals t2g orbital are filed

the t2g(π*) orbitals contain one electron, the eg(σ*) orbitals are empty

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the ground electronic configuration of a d1complex gives rise to only one state (2T2g)

and the excited electronic configuration also has one state (2Eg)

only one d–d band may be present in the spectrum of [Ti(H2O)6]3+

In a state diagram it Corresponds to the 2T2g 2Egtransition

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complexes containing more than one d-electron, the situation is complicated by the interelectronic repulsionsthe ground-electronic configuration of a d2 (3F) octahedral complex give rises to the 3T1g ,3T2g

3A2g , state (field free ion)absorption bands could then arise from the transitions3T1g 3T2g, 3T1g 3A2g, and

3T1g 3T1g(P)

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Therefore, three transitions correspond to the t2g

2 eg2 promotion (Transitions from the

ground state 3T1g to each one of the excited states described above are symmetry forbidden ) a d2 system, such as [V(H2O)6]3+ only two transition are allowed3T1g (t2g

2) 3T2g(t2geg) and 3T1g(t2g

2) 3T1g(t2geg) (give bands of sufficient intensity )

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Charge-transfer bands

the movement of electrons between orbitals that are predominantly ligand in character and orbitals that are predominantly metal in characterhigh intensity and the sensitivity of their energies to solvent polarity.electron migrates between orbitals that are predominantly ligand in character and orbitals that are predominantly metal in character

Charge Transfer Bands

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LMCTobserved in the visible region of the spectrum when the metal is in a high oxidation state and ligands contain nonbonding electrons.In octahedral complex we may distinguish four types of LMCTπL πM*(t2g), πLσM* (eg) ,σL πM* t2g) and σL σM*(eg)

[IrCl6]2-

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MLCT transitionslikely to happen in complexes with central atoms having small ionization potentials and ligands with easily available empty π*ligands such as CN–, CO, SCN– etc MLCT transitions will be favored when the metal has a low oxidation statebands in the visible range of [Ru(bpy)3]2+ due to MLCT while [Ru(bpy)3]3+ is due to LMCT

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ion-pair charge-transfer the ion pairs formed by a coordinatively saturated complex cation and a polarizable anion-like iodide .ion-pair charge-transfer bands are due to intermolecular CT transitions from the anion to the antibonding d- orbitals of the central metal.

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The intensity of these bands depends on the formation constant of the ion pair and on the concentration of the two ions. Intermolecular CT transitions of the inverse type (i.e. from the complex to an outer species) is called CTTS.exhibited by some negative complex ions such as [Fe(CN)6]4-.

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LLCT Or (Intra-Ligand) Bands due to transitions between two MOs both of which are principally localized on the ligand system .such bands may be found at relatively low energy in complexes containing ligands which have π-systems of their ownAromatic Ligands such as bipyridine and phenanthroline belong to this group

Effect of Solvent Polarity on CT Spectra

• only occurs if the species being studied is an ion pair

• The position of the CT band is reported as a transition energy and depends on the solvating ability of the solvent

• Polar solvent molecules align their dipole moments maximally or perpendicularly with the ground state or excited state dipoles

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Both the ground state and the excited state are neutral. When both the ground state and the excited state are neutral a shift in wavelength is not observedNo change occurs. Like dissolves like and a polar solvent won’t be able to align its dipole with a neutral ground and excited state.

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The excited state is polar, but the ground state is neutral It will align its dipole with the excited state and lower its energy by solvation. This will shift the wavelength to higher wavelength and lower frequency.

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The ground state and excited state is polarthe polar solvent will align its dipole moment with the ground stateMaximum interaction will occur and the energy of the ground state will be loweredThe dipole moment of the excited state would be perpendicular to the dipole moment of the ground state, since the polar solvent dipole moment is aligned with the ground stateThis interaction will raise the energy of the polar excited state

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The ground state is polar and the excited state is neutralthe ground state is polar the polar solvent will align its dipole moment with the ground state Maximum interaction will occur and the energy of the ground state will be loweredthe excited state is neutral no change in energy will occur

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• Like dissolves like and a polar solvent won't be able to align its dipole with a neutral excited state.

• Overall you would expect an increase in energy because the ground state is lower in energy (decrease wavelength, increase frequency, increase energy).

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• Conclusion

• the excited state properties of coordination and organometallic compounds play important roles in many research fields, including nanotechnology, solar energy conversion, and environmental issues.

• We hope that the present article can be useful as a source of information and a possible starting point for future developments. For a related chapter in this Comprehensive, we refer to