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The spectrochemical series

H&C p. 721

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p-bonding

The spectrochemical series cannot beexplained by crystal field theory or a s-bondingLFT model

F- should have the greatest charge density and soproduce the highest splitting

CO is not charged, so should have the lowestcharge density and so produce the smallest

splitting

Yet CO is a strong field ligand and F- a weakfield ligand

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Effect of p-bonding

The answer is p-bonding

From the ligand to the metal, e.g. halide ions

From the metal to the ligand, p-back-bonding

p-bonding cannot be described by anelectrostatic model such as crystal field theory.

Need ligand field theory.

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p-bonding between CO and a metal

There are TWELVE availableligand p-orbitals since wehave 6 ligands each of whichhas 2 p* orbitals

one in the plane of the board

the other out of the plane

These give TWELVEsymmetry based

combinations with symmetrylabels

t1g, t1u, t2u, and t2g

H&C p. 785

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Interaction with the s-bonding MOs

The ligand p-bonding combinations interact with thebonding MOs of the s-bond picture alteringthe MOpicture.

The s-bonding MOs span a1g, eg, and t1u. Of theseonly t1u can interact with the corresponding ligand p-bonding orbital set.

The t2g metal orbital set (which was non-bondingin thes-bond picture)now has a ligand p-bondingcombination with which it can interact, so is no longernon-bonding.

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The t2g ligand orbital set interacts withthe non-bonding metal t2g d-orbitals

H&S p. 567

ligand p-orbitals MO bonding MO anti-bondingcombination

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Why does it matter?

explains the spectrochemical series

origin of the 18-electron rule

tells us when the rule will be obeyed

tells us when the rule will not be obeyed

tells us how to deal with geometries other thanoctahedral, e.g. square planar.

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Effect of a p-donor interaction on the

HOMO and LUMO and size of Doct

The donor orbitals must be filledhow else could they donate anelectron pair?

Filled orbitals, are (relatively) low inenergy

Interact with the s-bonding molecularorbitals (off-screen and can ignore)and the metal t2g set the new t2g MOs contain the electron

pairs from the ligand p-donor setforming the p-bond

the new t2g* MOs contain any metald-electrons

Doct is now from t2g* to eg* and issmaller

S&A p. 238-240

**

t2g

t2g*

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Effect of a p-acceptor interaction on the

HOMO and LUMO and size of Doct

The acceptor orbitals must beempty how else could theyaccept an electron pair?

Empty orbitals, therefore(relatively) high in energy

Interact with the metal t2g set

the new t2g MOs contain any

metal d-electrons forming thep

-back-bond

the new t2g* MOs are above eg*

Doct is now from t2g to eg* and islarger

S&A p. 238-240

* eg*

t2g

t2g*

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The spectrochemical series

p-bonding interactions can explain the ordering ofligands in the spectrochemical series

p-acceptor ligands are strong field

p-donor ligands are weak field

H&C p. 721

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The 18-electron rule in ML6

In the absence of p-bonding 6 bonding orbitals

Complex needs12 electrons to fillthem and achieve lowest energy

e.g. [W(Me)6] (12e-)

3 non-bonding orbitals so system doesntcare if theyre filled or not up to 18 electrons allowed

[Mn(H2O)6]2+ (17e-)

[Fe(H2O)6]2+ (18e-)

2 weakly anti-bonding orbitals areaccessible At a push complex canaccommodate

up to 22 electrons [Ni(en)3]

2+ (20e-)

S&A p. 238-240

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The 18-electron rule in ML6: Effect of a p-

acceptor

t2g are now bonding

3 additional bondingorbitals

Complex needs18electrons to fill allbonding MOs andachieve lowest energy

e.g. [W(CO)6] (18e-)

S&A p. 238-240

* eg*

t2g

t2g*

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Square planar complexes

Crystal field splitting can be derived byconsidering electrostatic repulsions between themetal d-electrons and the ligand donor pair

Ligand field theory MO diagram can be derivedusing symmetry

ML

LL

L

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Metal dx2-y2 orbital overlaps withligand orbital set having b1g symmetry

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Square planar complexes and the 18electron rule

Need 4 s-bonding orbitals,

symmetry considerationsmake these involve the s, px,py and dz2, dx2-y2 orbitals

4 metal-dbasedbonding/non-bonding/weaklyanti-bonding orbitals orbitals

Total 8 bonding/non-bonding orbitals

Therefore need 16electrons

Miessler & Tarr p 434

Metal d-orbitals

Metal-ligandbondingorbitals

D