Chm2911 Topic 1: Properties of Transition Metal Complexes

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[Cr(CO)6] Cr gives 6 electrons. 6e- + 6e- + 6e- = 18e-

C gives 6 electrons. ∴ complex is stable.

O gives 6 electrons.

CRYSTAL FIELD THEORY

Splitting of d orbitals leads to an overall lowering of energy and consequently the complex is more stable than it was prior to splitting, accounting for the stability of most complexes.

The five degenerate d orbitals are split into two sets; 3 t2g orbitals of lower energy and 2 eg orbitals of higher energy. This energy separation is labelled Δoct. The t2g orbitals are lower in energy by 2/5Δoct, and the eg orbitals are higher in energy by 3/5Δoct.[pic 7]

The orbitals are shown schematically and can be filled one of two ways:

High spin: occurs when Δoct is small and are preferentially unapired.

Low spin: occurs when Δoct is large and too much energy is required to promote the fourth electron into the eg orbital. Here, electrons are preferentially paired.

We can calculate the overall stabilisation energy for any configuration via the equation:[pic 8]

CFSE = #e- in t2g(-2/5Δoct) + #e- in eg(3/5Δoct).

There are an array of factors effecting the crystal field splitting, including:

- Identity of the metal: crystal field splitting (Δ) is about 50% greater for the second transition series compared to the first, whereas the third species is about 25% greater than the second. Hence, there is a small increase in the splitting along each series as there is an increase in Δoct down groups.

- Oxidation state of the metal: the higher the oxidation state, the greater the crystal field splitting.

- Number of ligands: crystal field splitting is greater for a large number of ligands.

- Nature of the ligands: the common ligands can be ordered on the basis of the effect that they have on the crystal field splitting. This listing is called the spectrochemical series. [pic 9]

Thus, the ligand that determines the value of the crystal field splitting is generally:[pic 10]

COLOUR

Values of Δoct are measured experimentally by UV-vis spectroscopy. For the simplest situation, a d1 configuration, there is one possible electron transition.

An absorption of electromagnetic energy causes the electron to shift to the eg orbital before returning to the ground state. Here, energy is released as thermal motion rather than as electromagnetic radiation. This transition occurs in the visible region of the electromagnetic spectrum and explains the colouring of complexes. We can convert wavenumbers to kJmol-1 via: [pic 11]

Δo = E = hvNo = hcNo/λ

where h is Planck’s constant, c is the speed of light, and No is Avogadro’s constant.

Many transition metal complexes are very slightly coloured and the absorption bands are weak due to such d-d transition being formally ‘forbidden’ by Laparte’s selection rule, which states that there must be a change in the value of the subsidiary quantum number, l.

Laparte’s selection rule only applies to molecules that have a centre of inversion and thus this rule is relaxed for tetrahedral complexes as they do not have a centre of inversion making them more colourful than octahedral complexes. [pic 12]

Ligand-metal charge transfer: intensely coloured metal complexes are due to ligand-metal charge transfer transitions when electrons move from the oxygen ligand onto the metal (p → d transitions). Metal-ligand charge transfers complexes also exist when the ligand has an empty p orbital that can accept electrons from the metal. [pic 13]

Spin selection rule: rule that states a change in spin multiplicity is forbidden. In d1, d4, d6, d9 complexes there is one absorption band; d2, d3, d7, d8 complexes there are three absorption bands; and in d5 complexes there is a series of very weak bands.

[pic 14]

EFFECT OF CO-ORDINATION NUMBER[pic 15]

The splitting diagram for a tetrahedral complex is inverted in comparison to the octahedral. With only four ligands instead of six, and the ligands not quite pointing directly at the three d orbitals, the crystal field splitting is much less than in the octahedral case, it is about 4/9 of Δoct. Due to the small orbital splitting, tetrahedral complex are almost all high spin and less stable than analogous octahedral complexes with 8d electrons tend to form square planar complexes. The configuration confers additional stability as Δsp is larger than Δoct. In square planar complexes the x2-y2 orbital is high energy and never filled with electrons. [pic 16]

JAHN-TELLER DISTORTION

Octahedral complexes of d9, low spin d7, and high spin d4 are often distorted due to unsymmetrically filled eg orbitals. This is common for Cu2+ complexes. The distortion increases the crystal field splitting and enhances stability. This distortion can lead to an elongated or compressed octahedron and is due to the unsymmetrically filled eg orbitals.

For d4, if the single electron in the eg orbital is in the z2 orbital there will be greater repulsion in the z direction than the x or y directions and the complex will be elongated. For elongation of the x or y axis the inverse process takes place (here electron is in the dx2y2 orbital). Similar arguments hold me for the d9 case. Which distortion happens is a matter of energetics but because axial elongation weakens four bonds, axial is more common.[pic 17]

CHELATE AND MACROCYCLIC EFFECTS

When Kf1 for the formation of a complex with a bidentate chelate ligand is compared with the value β2 for the corresponding bis complex, I tis found that the former is generally larger.

[pic 18]

Two similar Cd-N bonds are formed in each case, yet the formation of the chelate-containing complex is distinctly more favourable. The greater stability of chelated complexes compared with their non-chelated analogous is called the chelate effect. This effect can be traced primarily to differences in reaction entropy between the analogous complexes in dilute solutions. The chelation reaction results in an increase

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