Transcript FACTORS AFFECT

METAL LIGAND
BONDING IN
TRANSITION METAL
COMPLEXES
VALENCE BOND THEORY
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.
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BASIS OF VALENCE BOND
THEORY
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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.
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.
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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.
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.
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SHAPES of d- ORBITAL
LIGANDS APPROACHING THE
BARE METAL ION
SPLITTING OF d-ORBITALS IN
OCTAHEDRAL COMPLEXES
HIGH SPIN COMPLEXES
LOW SPIN COMPLEXES
EXAMPLES OF HIGH AND LOW
SPIN COMPLEXES
VARIATION OF CFSE IN OCTAHEDRAL WITH
DIFFERENT LIGANDS
CFSE IN HIGH SPIN COMPLEXES
CFSE IN LOW SPIN COMPLEXES
TETRAHEDRAL COMPLEXES
SPLITTING OF d-ORBITALS IN
TETRAHEDRAL COMPLEXES
SQUARE PLANAR COMPLEXES
SPLITTINGIN SQUARE PLANAR
COMPLEXES
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.
 THE
METAL ION WITH HIGHER
OXIDATION STATE CAUSE LARGE
CRYSTAL FIELD SPLITTINGTHAN IS
DONE BY THE ION WITH LOWER
OXIDATION STATE.
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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.
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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.
LIMITATIONS OF CRYSTAL FIELD
THEORY
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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.