METAL LIGAND BONDING IN TRANSITION METAL COMPLEXES

1. METAL LIGAND BONDING IN TRANSITION METAL COMPLEXES

2. This theory was developed by Linus Pauling. According to this theory the bonding in metal complexes arises when a filled ligand orbital containing a pair of electrons overlaps with a vacant hybrid orbital on the metal atom. The VBT assumes the bonding between the metal atom and ligands to be purely covalent. VALENCE BOND THEORY

3. BASIS OF VALENCE BOND THEORY • The central metal atom in the complexes makes available a number of empty orbitals for the formation of coordinate bonds with suitable ligands. • The appropriate atomic orbitals of the metal hybridise to give a set of new orbitals of equivalent energy, called hybrid orbitals. • Each ligand has at least one orbital containing a lone pair of electrons. • The empty hybrid orbitals of metal ion overlap with the filled orbitals of the ligand to form a covalent sigma bond. the bond is called coordinate bond.

4. LIMITATIONS OF VALENCE BOND THEORY • IT PROVIDES ONLY QUALITATIVE EXPLANATIONS FOR COMPLEXES. • IT DOES NOT EXPLAIN THE VARIATIONS OF MAGNETIC MOMENT WITH TEMPERATURE. • IT DOES NOT TAKE INTO ACCOUNT THE SPLITTING OF d –ENERGY LEVELS. • IT DOES NOT PREDICT THE RELATIVE STABILITIES OF DIFFERENT STRUCTURES.

5. CRYSTAL FIELD THEORY • THE C.F.T THEORY WAS DEVELOPED BY H.BETHE AND V. BLECK.THIS THEORY CONSIDERS THE BOND BETWEEN METAL ION AND THE LIGAND AS PURELY ELECTROSTATIC.

6. BASIS OF CRYSTAL FIELD THEORY • THE TRANSITION METAL ION IS SURROUNDED BY THE LIGAND WITH LONE PAIR OF ELECTRONS. • ALL TYPES OF LIGANDS ARE REGARDED AS POINT CHARGES. • THE INTERACTIONS BETWEEN METAL ION AND THE NEGATIVE END OF ANION IS PURELY ELECTROSTATIC. • THE LIGANDS SURROUNDING THE METAL ION PRODUCE ELECTRICAL FIELD WHICH INFLUENCES THE ENERGY OF d ORBITALS OF CENTRAL METAL ION.

7. SHAPES of d- ORBITAL

8. SHAPES OF d ORBITALS

11. LIGANDS APPROACHING THE BARE METAL ION

12. SPLITTING OF d-ORBITALS IN OCTAHEDRAL COMPLEXES

13. HIGH SPIN COMPLEXES

14. LOW SPIN COMPLEXES

15. EXAMPLES OF HIGH AND LOW SPIN COMPLEXES

17. VARIATION OF CFSE IN OCTAHEDRAL WITH DIFFERENT LIGANDS

18. CFSE IN HIGH SPIN COMPLEXES

19. CFSE IN LOW SPIN COMPLEXES

20. TETRAHEDRAL COMPLEXES

22. SPLITTING OF d-ORBITALS IN TETRAHEDRAL COMPLEXES

26. SQUARE PLANAR COMPLEXES

27. SPLITTINGIN SQUARE PLANAR COMPLEXES

30. FACTORS AFFECTING MAGNITUDE OF CRYSTAL FIELD SPLITTING NATURE OF THE LIGAND- SMALL LIGANDS APPROACH THE LIGANDS EASILY, SO THEY CAN CAUSE GREAT CRYSTAL FIELD SPLITTING. LIGANDS CONTAINING EASILY POLARISABLE ELECTRON PAIR WILL BE DRAWN MORE EASILY TO THE METAL ION.

31. OXIDATION STATE OF METAL ION THE METAL ION WITH HIGHER OXIDATION STATE CAUSE LARGE CRYSTAL FIELD SPLITTINGTHAN IS DONE BY THE ION WITH LOWER OXIDATION STATE.

32. TYPES OF d- ORBITALS THE EXTENT OF C.F.S FOR SIMILAR COMPLEXES OF METAL IN THE SAME OXIDATION STATE INCREASED BY ABOUT THIRTY TO FIFTY PERCENT ON GOING FROM FIRST TRANSITION SERIES TO THIRD TRANSITION SERIES. THE INCREASE IS ALMOST SAME AMOUNT AS GOING FROM SECOND TRANSITION SERIES TO THIRD TRANSITION SERIES. THIS MAY BE EXPLAINED ON THE BASIS THAT 4d ORBITAL IN COMPARISON TO 3d ORBITAL ARE BIGGER IN SIZE.AS A RESULT 4d ORBITAL INTERACT STRONGLY WITH LIGAND.

33. GEOMETRY OF THE COMPLEX CRYSTAL FIELD SPLITTING ENERGY OF TETRAHEDRAL COMPLEXES IS NEARLY HALF THE VALUE FOR OCTAHEDRAL COMPLEXES. IN OTHER WORDS, THE VALUE OF C.F.S.E FOR TETRAHEDRAL COMPLEXES IS SMALL AS COMPARED TO PAIRING ENERGY P.THE TETRAHEDRAL COMPLEXES ARE THEREFORE MOSTLY HIGH SPIN COMPLEXES.

34. LIMITATIONS OF CRYSTAL FIELD THEORY • It does not take into account the partial covalent character of metal-ligand bond. • It does not consider multiple bonding between metal ion and ligand. • It does not explain the relatives strengths of ligands. • It does not explain the charge transfer bands. • It considers only d-orbitals of metal ions.

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