Crystal Field Theory - Notes from inorganic section of course PDF

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Notes from inorganic section of course...

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Crystal Field Theory Explain Crystal Field Theory

CFT  

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Crystal field theory is based upon orbitals but does not go fully into all of the molecular orbitals and potential combinations etc. (That is called Ligand Field Theory–vide supra) With CFT we are just going to consider the d-orbitals and how they can interact with ligands o d-orbitals are highly directional

most TM compounds adopt an octahedral geometry with six ligands about the metal centre o Overall this is a highly energetically favourable process (opposite charges attract)

Bonding in Transition Metal Complexes   

Crystal field theory of bonding in octahedral coordination complexes In the absence of any ligands, the five d-orbitals are degenerate. An octahedral metal complex, [M(L)6]n+, can be viewed as six negative point charges approaching a metal cation (overall, a very stabilizing interaction).

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In presence of ligands, degeneracy is removed. That means the d-orbitals have different energies or they are split o The net result is the colouration of transition metal complexes.

Crystal Field Theory: d-Orbital Splitting  

Each ligand lone pair about the metal centre represents a negative point charge directed towards the metal atom. These ‘charges’ coulombically repel electrons in those d-orbitals closest to them, leading to destabilisation (i.e. an increase in energy of those d-orbitals that are close)

Explain the impacts of crystal theory on the degeneracy of the d-orbitals Construct an Orbital Diagram to show d-orbitals in an octahedral complex

Spatial Placement of the Orbitals 

Two of the d-orbitals lie with lobes directed towards the ligands. These become destabilised

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Three of the d-orbitals have lobes lying between the ligands. These are also destabilised but less than the e g orbitals o These orbitals are labelled the t 2g orbitals

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Due to the different spatial arrangement, electrons in the e g orbitals are destabilised more by the octahedral (Oh) ligand field than those in the t2g orbitals. The energy difference between the two types of d-orbital is known as the crystal field splitting ( Δg)

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CFT- Splitting

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Magnitude of Δo (crystal field splitting energy in an octahedral field) depends on: o Nature of the ligand o Oxidation state of the metal (Δo increases with oxidation state) o Position of the transition metal in the periodic table (Δ o increases down a group)

Crystal field theory: octahedral field notes d x − y , d z orbitals point directly at ligands  2

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2

2

d xy , d xz , d yz

orbitals point “in between” ligands

Ligands are negatively charged (or at least partially) o repulsion between negatively charged ligands and d-electrons

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Proximity of d x − y , d z

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to d xy , d xz , d yz (t2g) Difference in energy called Δo Δo is significantly larger for the 2nd transition metal series (Y to Ag) and 3rd transition metal series (La to Au) o complexes of 2nd and 3rd series metals are almost invariably low spin

2

2

2

orbitals (eg) to negatively charged ligands makes them higher in energy compared

Factors Affecting Δ 

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Oxidation State of the metal: Δo increases with an increase in the oxidation state of the metal. o As electrons are removed from the metal, the charge on the ion becomes more positive and the ion becomes smaller. o The ligands experience greater attraction resulting in greater repulsion ∴ greater splitting The ligand The number of ligands and the geometry of the complex

Spectrochemical series - order of the magnitude of their crystal field splitting energies

C-donors > N-donors > O-donors > Halides  

Ligands I-to OH2 (Halides and O-doors) as weak-field ligands Ligands NH3 to CO (N-& C-donors) as strong-field ligands

Use the spectrochemical series to predict Δ

Fill d-orbitals in an Octahedral Complex

Electron Filling in CFT 

When filling the d-orbitals in CFT you only consider the d-electrons that the metal has

High Spin and Low Spin   

For d-electron counts 4-7 we have to consider the ligands and their effect on ∆ o Consider the cost of pairing 2-electrons together in an orbital Systems where ∆ is large: we pair the electrons in t2g before populating the eg Systems where ∆ is small: we populate the eg before populating the t2g

High-Spin & Low-Spin  Weak-field ligands give High-Spin complexes  Strong-field ligands give Low-Spin complexes

D8-10 

For d-electron counts of 8-10 we do not have to worry about high/low-spin...