PowerPoint presentation on the topic ORGEL DIAGRAM

This article contains informative study regarding orgel diagram with attached hand made picture to understand better.

1. Topic:- Orgel diagram Name:- Rifat Riyaz M.Sc. 1 st semester Roll no. :- 2310014175011 Mentor name:- Prof. V.K.Rai sir Chemistry presentation

2. ORGEL DIAGRAM

3. Introduction Introduced by “ Laslie orgel ” . It is a diagrammatical representation of electron absorption spectra of transition metal complexes. Particularly useful in interpretation of only spin allowed electronic transition. Spin allowed electronic transition occurs between the two energy levels which have same spin multiplicity. It is given only for tetrahedral and high spin octahedral complexes of transition metal atom. Almost all molecules of complex occupy ground state energy levels therefore ,this diagram shows ground state and only those excited states that have same spin multiplicity as of ground state.

4. In an orgel diagram the parent term ( P, D or F) in the presence of no ligand is shown in the centre and the different energy levels arising from the parent term on either side. Different energy level arises in the presence of ligand field and are represented by Mulliken’s symbols. These Mulliken’s symbols have the same spin multiplicity as the unsplitted parent term. The parent term used for different configurations in spin allowed electronic transition are given below:- Terms used in orgel diagram Configuration Energy terms d 1 , d 9 2 D d 2 ,d 8 3 F , 3 P d 3 , d 7 4 F , 4 P d 4 , d 6 5 D d 5 6 S

5. Splitting of parent term in octahedral and tetrahedral ligand field The parent term under ligand field are splitted into different energy levels which are represented by Mulliken’s symbols as given below:- Spectroscopic terms Splitted forms Mulliken’s symbol Octahedral field (Oh) Tetrahedral Field (Td) S Remain unsplitted (1) A 1g A 1 P Remain unsplitted (3) T 1g T 1 D 5 orbital→2 sets (3+2) T 2g + E g T 2 + E F 7orbital→3 sets (3+3+1) T 1g + T 2g + A 2g T 1 + T 2 + A 2

6. ORGEL DIAGRAM “F” Orgel diagram “D” Orgel diagram ( for configuration having D term as a ground state) For d 1 ,d 4 , d 6 and d 9 systems. ( for configuration having F term as a ground state) For d 2 ,d 43 , d 7 and d 8 systems. Note:- For d⁵ configuration it is a special case which shows spin forbidden transition. The d⁵ configuration have ⁶S ground state which do not split in ligand field and it has no other excited state of same spin multiplicity hence in this case only spin forbidden transitions are possible.

7. “D” Orgel diagram It is given for d 1 ,d 6 ,d 4 and d 9 system. For the “D” orgel diagram, left side contains d 1 and d 6 octahedral and d 4 and d 9 tetrahedral complexes. The right side contains d 4 and d 9 octahedral, and d 1 and d 6 tetrahedral complexes. The lowest energy electronic transition or absorption band on the left side of spectrum is T 2 →E ,while on the right side of spectrum it is E→T 2 . The subscript “g” is used if the same diagram is used to generalize spectra of octahedral complexes. Fig:-1)-Orgel diagram for d 1 ,d 4 ,d 6 and d 9 complexes in octahedral (Oh) and tetrahedral (Td) crystal fields

8. “F” Orgel diagram Fig:2):- Orgel diagram for d 2 , d 3 , d 7 and d 8 complexes in octahedral (Oh) and tetrahedral (Td) crystal fields. It is given for d 2 , d 3 , d 7 and d 8 systems. For the “F” Orgel diagram, left side contains d 2 and d 7 tetrahedral and d 3 and high spin d 8 octahedral complexes. The right side contains d 3 and d 8 tetrahedral and d 2 and high spin d 7 octahedral complexes. The lowest energy absorption band on the left side of spectrum is of A 2 (F)→T 2 (F) transition, while on the right side of the spectrum it is of T 1 (F)→T 2 (F) transition . It is clear from given diagram, that there are four states of same spin multiplicity, two T 1 states, one T 2 state and one A 2 state. The spin multiplicity is omitted as it is a generalized diagram. The subscript “g” is used if the same diagram is used to generalize spectra of transition metal complexes in octahedral field.

9. Application of orgel diagram to electronic spectra of transition metal complexes The electronic Spectra of transition metal complexes can be obtained by the orgel diagram. With the help of orgel diagram, it is easy to determine the number and type of electron transition of d-d origin. Orgel diagram and electronic spectra of d 2 and d 8 octahedral complexes Fig:3):-Orgel diagram for d 2 metal ion in octahedral field Fig:4):-Orgel diagram for d 8 metal ion in octahedral field The ground state term symbol for d² and d⁸ complexes is ²F, but the splitting pattern is opposite for each other. The energy order of Mulliken’s state in d⁸ configuration complexes will be just inverse of what is in d² configuration. The ³F ground state term splits in ³A 2g (F), ³T 2g , and ³T 1g in octahedral field. The excited state with same spin multiplicity is ³P which transforms to ³T 1g (P) in octahedral field. For d² metal ion:- The order of energy level in ligand field is ³T 1g (F), ³T 2g (F), ³T 1g (P) and ³A 2g (F). Thus. There should be three absorption band due to ³T 1g (F)→³T 2g (F), ³T 1g (F)→³T 1g (P) and

10. ³T 1g (F)→³A 2g (F) transitions. But actually in d² metal ion ,only two absorption bands are observed. The third electronic transition ³T 1g (F)→³A 2g (F) involve simultaneous excitation of both electrons from t 2g to e g orbital and hence is forbidden. Fig:5):-Electronic spectra of [V(H 2 O) 6 ]³+ complex (d² metal complex. Fig:6):- Electronic spectra of [Ni(H 2 O) 6 ]²+ complex For d⁸ metal ion in octahedral field - For e.g. ,the complex ion [V(H 2 O) 6 ]³ + is d² metal complex which shows absorption band at 17,200 cm -1 and 25,600 cm -1 and it’s aqueous solution is green in colour. The order of energy level in ligand field is ³A 2g (F), ³T 2g (F), ³T 1g (F) and ³T 1g (P). There are three absorption bands due to ³A 2g (F)→³T 2g (F), ³A 2g (F)→³T 1g (F) and ³A 2g (F)→³T 1g (P) electronic transitions. The term ³T 1g (F) and ³T 1g (P) do not cross each other because they are of same symmetry For e.g. ,The complex ion [Ni(H 2 O) 6 ]² + is a d⁸ complex ion which shows absorption band at 8,700cm -1 ,14,500 cm -1 and 25,300 cm -1 and is blue in colour in aqueous medium.

11. ** Hence we construct orgel diagram for any transition metal configuration and their electronic spectra in the same manner but in accordance with certain rules also which are given below:- Rules to be followed:- The ground state and excited state of same spin multiplicity should be taken Same spin multiplicity are written for Mulliken’s symbols obtained by splitting of terms in crystal field. Lines of the same term diverge due to non crossing rule . For different term the lines of same symmetry do not cross each other but of opposite symmetry can cross each other . Subscript “g” is used to all term energy levels if the metal ion is subjected to octahedral field potential and it is omitted in case of tetrahedral field potential. All the possible transition should be shown in the diagram.

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