METAL LIGAND METAL LIGAND BONDING IN BONDING IN

TRANSITION METAL TRANSITION METAL COMPLEXESCOMPLEXES

VALENCE BOND THEORYVALENCE BOND THEORY

This theory was developed by Linus This theory was developed by Linus Pauling. According to this theory the Pauling. According to this theory the bonding in metal complexes arises when a bonding in metal complexes arises when a filled ligand orbital containing a pair of filled ligand orbital containing a pair of electrons overlaps with a vacant hybrid electrons overlaps with a vacant hybrid orbital on the metal atom. orbital on the metal atom.

The VBT assumes the bonding between the The VBT assumes the bonding between the metal atom and ligands to be purely metal atom and ligands to be purely covalent.covalent.

BASIS OF VALENCE BOND BASIS OF VALENCE BOND THEORYTHEORY

The central metal atom in the complexes makes The central metal atom in the complexes makes available a number of empty orbitals for the available a number of empty orbitals for the formation of coordinate bonds with suitable formation of coordinate bonds with suitable ligands.ligands.

The appropriate atomic orbitals of the metal The appropriate atomic orbitals of the metal hybridise to give a set of new orbitals of hybridise to give a set of new orbitals of equivalent energy, called hybrid orbitals.equivalent energy, called hybrid orbitals.

Each ligand has at least one orbital containing a Each ligand has at least one orbital containing a lone pair of electrons.lone pair of electrons.

The empty hybrid orbitals of metal ion overlap The empty hybrid orbitals of metal ion overlap with the filled orbitals of the ligand to form a with the filled orbitals of the ligand to form a covalent sigma bond. the bond is called covalent sigma bond. the bond is called coordinate bond.coordinate bond.

LIMITATIONS OF VALENCE LIMITATIONS OF VALENCE BOND THEORYBOND THEORY

IT PROVIDES ONLY QUALITATIVE IT PROVIDES ONLY QUALITATIVE EXPLANATIONS FOR COMPLEXES.EXPLANATIONS FOR COMPLEXES.

IT DOES NOT EXPLAIN THE VARIATIONS OF IT DOES NOT EXPLAIN THE VARIATIONS OF MAGNETIC MOMENT WITH TEMPERATURE.MAGNETIC MOMENT WITH TEMPERATURE.

IT DOES NOT TAKE INTO ACCOUNT THE IT DOES NOT TAKE INTO ACCOUNT THE SPLITTING OF d –ENERGY LEVELS.SPLITTING OF d –ENERGY LEVELS.

IT DOES NOT PREDICT THE RELATIVE IT DOES NOT PREDICT THE RELATIVE STABILITIES OF DIFFERENT STRUCTURES.STABILITIES OF DIFFERENT STRUCTURES.

CRYSTAL FIELD THEORYCRYSTAL FIELD THEORY

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

BASIS OF CRYSTAL FIELD BASIS OF CRYSTAL FIELD THEORYTHEORY

THE TRANSITION METAL ION IS THE TRANSITION METAL ION IS SURROUNDED BY THE LIGAND WITH LONE SURROUNDED BY THE LIGAND WITH LONE PAIR OF ELECTRONS.PAIR OF ELECTRONS.

ALL TYPES OF LIGANDS ARE REGARDED AS ALL TYPES OF LIGANDS ARE REGARDED AS POINT CHARGES.POINT CHARGES.

THE INTERACTIONS BETWEEN METAL ION THE INTERACTIONS BETWEEN METAL ION AND THE NEGATIVE END OF ANION IS AND THE NEGATIVE END OF ANION IS PURELY ELECTROSTATIC.PURELY ELECTROSTATIC.

THE LIGANDS SURROUNDING THE METAL THE LIGANDS SURROUNDING THE METAL ION PRODUCE ELECTRICAL FIELD WHICH ION PRODUCE ELECTRICAL FIELD WHICH INFLUENCES THE ENERGY OF d ORBITALS INFLUENCES THE ENERGY OF d ORBITALS OF CENTRAL METAL ION.OF CENTRAL METAL ION.

SHAPES of d- ORBITALSHAPES of d- ORBITAL

LIGANDS APPROACHING THE LIGANDS APPROACHING THE BARE METAL IONBARE METAL ION

SPLITTING OF d-ORBITALS IN SPLITTING OF d-ORBITALS IN OCTAHEDRAL COMPLEXESOCTAHEDRAL COMPLEXES

HIGH SPIN COMPLEXESHIGH SPIN COMPLEXES

LOW SPIN COMPLEXESLOW SPIN COMPLEXES

EXAMPLES OF HIGH AND LOW EXAMPLES OF HIGH AND LOW SPIN COMPLEXESSPIN COMPLEXES

VARIATION OF CFSE IN OCTAHEDRAL WITH VARIATION OF CFSE IN OCTAHEDRAL WITH DIFFERENT LIGANDSDIFFERENT LIGANDS

CFSE IN HIGH SPIN COMPLEXESCFSE IN HIGH SPIN COMPLEXES

CFSE IN LOW SPIN COMPLEXESCFSE IN LOW SPIN COMPLEXES

TETRAHEDRAL COMPLEXESTETRAHEDRAL COMPLEXES

SPLITTING OF d-ORBITALS IN SPLITTING OF d-ORBITALS IN TETRAHEDRAL COMPLEXESTETRAHEDRAL COMPLEXES

SQUARE PLANAR COMPLEXES SQUARE PLANAR COMPLEXES

SPLITTINGIN SQUARE PLANAR SPLITTINGIN SQUARE PLANAR COMPLEXESCOMPLEXES

FACTORS AFFECTING MAGNITUDE FACTORS AFFECTING MAGNITUDE OF CRYSTAL FIELD SPLITTINGOF CRYSTAL FIELD SPLITTING

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

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

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

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

LIMITATIONS OF CRYSTAL FIELD LIMITATIONS OF CRYSTAL FIELD THEORYTHEORY

It does not take into account the partial covalent It does not take into account the partial covalent character of metal-ligand bond.character of metal-ligand bond.

It does not consider multiple bonding between It does not consider multiple bonding between metal ion and ligand.metal ion and ligand.

It does not explain the relatives strengths of It does not explain the relatives strengths of ligands.ligands.

It does not explain the charge transfer bands.It does not explain the charge transfer bands. It considers only d-orbitals of metal ions.It considers only d-orbitals of metal ions.