photo chemistry of ligand in coordination compound
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the photo chemistry of ligand field is very important to have an idea for the intrinsic properties of different coordination compound, and the electronic properties such as, LMCT,LLCT, MLCH etc..........
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photo chemistry of ligand in coordination compound
- 1. Addis Ababa University Department of Chemistry PhD Program Photochemistry of Ligand Field Transition presenter: Masresha Amare Adviser: Prof V.J .T Raju Dr. Yonas Chebude
- 2. 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
- 3. Cont……… Electronic excitation change of molecular orbital occupancy increased energy change of bonding characteristics and possibly geometry change of charge distribution possible changes of resultant electron spin, orbital symmetry
- 4. Cont…………. AB* Fates of photoexcited species Physical quenching AB BA Isomerization A + B Dissociation AB+ + e- Ionization Luminescence AB + hν AB + CD‡ Intermolecular energy transfer AB† Intramolecular energy transfer (radiationless transition) AB + E or ABE Direct reaction AB⋅ + + E⋅ - or AB⋅ - + E⋅ + Charge transfer + M + CD + E
- 5. Electronic Transitions Three types of electronic transitions may be distinguished transitions 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
- 6. Cont………… Octahedral complexes Cr(III) ground state of these complexes,4 A2g, belongs to the configuration t2g 3 . Spin allowed 4 A2g(t2g 3 ) → 4 T2g(t2g 2 eg1 ) and 4 A2g → 4 T1g correspond to the promotion of an electron from the t2g to eg orbitals.
- 7. Cont………….
- 8. Cont…………..
- 9. Cont………. Effect of T2g → eg (1) an increase in the metal–ligand repulsion some lengthening of metal-ligand distances breaking of a metal-ligand bond rearrangement of the molecule toward a more-stable structure (isomerization
- 10. Cont………….. (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)
- 11. Cont……
- 12. Cont………… Transitions between MOs mainly localized on the ligands and MOs mainly localized on the central metal. This transition are called CT or optical electron transfer LMCT or MLCT, transitions can occur.
- 13. Cont……….. 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.’
- 14. Absorption Bands
- 15. 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
- 16. Cont……… the ground electronic configuration of a d1 complex gives rise to only one state (2 T2g) and the excited electronic configuration also has one state (2 Eg) only one d–d band may be present in the spectrum of [Ti(H2O)6]3+ In a state diagram it Corresponds to the 2 T2g → 2 Egtransition
- 17. Cont………….
- 18. Cont……….. complexes containing more than one d- electron, the situation is complicated by the interelectronic repulsions the ground-electronic configuration of a d2 (3 F) octahedral complex give rises to the 3 T1g,3 T2g 3 A2g, state (field free ion) absorption bands could then arise from the transitions3 T1g → 3 T2g, 3 T1g → 3 A2g, and 3 T1g → 3 T1g(P)
- 19. Cont……….. Therefore, three transitions correspond to the t2g 2 → eg 2 promotion (Transitions from the ground state 3 T1g 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 allowed 3 T1g (t2g 2 ) → 3 T2g(t2geg) and 3 T1g(t2g 2 ) → 3 T1g(t2geg) (give bands of sufficient intensity )
- 20. Cont…………..
- 21. Charge-transfer bands the movement of electrons between orbitals that are predominantly ligand in character and orbitals that are predominantly metal in character high 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
- 23. Cont…………. LMCT observed 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 LMCT πL → πM*(t2g), πL→σM*(eg) ,σL→ πM* t2g) and σL → σM*(eg) [IrCl6]2-
- 24. Cont……….. MLCT transitions likely 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 state bands in the visible range of [Ru(bpy)3]2+ due to MLCT while [Ru(bpy)3]3+ is due to LMCT
- 25. Cont……… 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.
- 26. Cont……….. 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- .
- 27. Cont………… 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 own Aromatic Ligands such as bipyridine and phenanthroline belong to this group
- 28. 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
- 29. Cont……………….. 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 observed No change occurs. Like dissolves like and a polar solvent won’t be able to align its dipole with a neutral ground and excited state.
- 30. Cont………. 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.
- 31. Cont……..
- 32. Cont……….. The ground state and excited 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 lowered The 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 state This interaction will raise the energy of the polar excited state
- 33. Cont………..
- 34. Cont…….. The ground state is polar and the excited state is neutral the 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 lowered the excited state is neutral no change in energy will occur
- 35. Cont………… • 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).
- 36. Cont………..
- 37. • 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
Editor's Notes
- Electronic excitation change of molecular orbital occupancy increased energy change of bonding characteristics and possibly geometry change of charge distribution possible changes of resultant electron spin, orbital symmetry
- A luminescent complex is one that re-emits radiation after it has been electronically excited. Fluorescence occurs when there is no change in multiplicity, whereas phosphorescence occurs when an excited state undergoes intersystem crossing to a state of different multiplicity and then undergoes radiative decay.
- electron transitions occurring between ‘metal’ d- orbital. the ground state of complexes is usually stable toward intramolecular oxidation–reduction processes. The angular rearrangement of the metal electrons can cause ligand substitution or isomerization reactions
- Factor Influencing CT excited states in Redox processes the stability of upper and lower oxidation states of the metal and ligands. the effective amount of CT induced by irradiation whether or not the CT is localized between one particular ligand and the metal (X M CT states in MLn-1X complexes) environnemental conditions (cage effect, solvent reactivity.)
- Depending on whether the excited electron is originally located on the ligands or on the central metal, ligand-to-metal or metal-to-ligand charge-transfer (LMCT or MLCT, respectively) transitions can occur.
- For example, the configuration d2in the field free ion V3+has these terms (in order of increasing energy):3F(ground state)1D,3P,1G, and 1S. In the octahedral symmetry, the free ion terms split as follows: 3F :3T1g (ground state) 3T2g , 3A2g 1D :1T2g,1Eg 3P :3T1g 1G :1A1g,1T2g,1T1g,1Eg 1S :1A1g
- Charge-transfer bands in the visible region of the spectrum (and hence contributing to the intense colours of many complexes) may occur if the ligands have lone pairs of relatively high energy (as in sulfur and selenium) or if the metal atom has low-lying empty orbitals.
- LMCT bands are usually in the UV region; however, they can extend to the visible spectral region, particularly for complexes containing highly reducing ligands such as I-, and Br– oxalate2– it is a common observation that the wavenumber of these bands decreases as the central ion becomes more oxidizing and the ligands more reducing. .
- where πL and σL are MOs mainly localized on the ligands, and πM* (t2g) and σM*(eg) are the MOs which receive the most important contribution from the two groups of metal d orbital note that in the low-spin d6octahedral complexes, such as those of Co(III) the ‘metal’ t2g orbitals are filled, so that the πL----πM* ( t2g) and σL--------- πM* (t2g) transitions cannot occur as a consequence, these complexes cannot exhibit LMCT bands at low energy
- Ligands such as CN–, CO, SCN–, and especially the conjugated carbon-containing molecules possess empty π* orbitals suited for such transitions. Regarding the central atoms, it is obvious that MLCT transitions will be favored when the metal has a low oxidation state .forexample, the bands in the visible range of [Ru(bpy)3]2+ (bpy=2,2-bipyridine) are due to MLCT transitions, while those of [Ru(bpy)]3+ are due to LMCT transitions.
- A typical example of an intermolecular CT band is given by the intense new band which appears in the spectrum of aqueous solutions of [Co (NH3)6]3+ salts when the concentration of iodide ion is increased. these types of CT bands have the same nature of CT bands found in supramolecular, compounds made of organic donor and acceptor subunits. Intermolecular CT transitions of the inverse type (i.e., from the complex to an outer species) are the so-called charge transfer-to-solvent (CTTS) transitions exhibited by some negative complex ions such as [Fe(CN)6]4-
- The first LLCT that was assigned was a near-UV band in the absorption spectra of [Be(bipy)(X2)] complexes.8The LLCT bands red-shifted in the order of the increasing reducing strength of the halide, consistent with an assignment of a halide orbital to bipy antibonding orbital charge transfer. some bands of complexes containing aromatic ligands, such as bipyridine, terpyridine, and phenanthroline, surely belong to this class. generally it is possible to identify bands as intra-ligand or CT in nature by observing their energy shifts in a series of complexes of a given ligand with a variety of different metals.
- more specifically, it should also be considered that: (1) the MLCT transitions increase the positive charge of the metal, so that a nucleophilic attack by outer ligands could be facilitated, and (2) in most LMCT transitions, the transferred electron goes into a metal orbital which points in the direction of the ligands (i.e., in a eg σ-antibonding orbital): a metal–ligand repulsion could then result, analogously to what happens in the case of the ligand-field excited state. moreover, CT transitions are expected to cause a strong change in the acid strength of complexes which contain ‘protogenic’ ligands, such as H2O, NH3, and cyano ligands.