Properties of coordination compounds Part 3 (3/3)

Stability of coordination compounds

http://wwwchem.uwimona.edu.jm/courses/IC10Kstability.html

Thermodynamic stability (equilibrium constant)

http://chimge.unil.ch/En/complexes/ressources/cpxenc.pdf

Equilibrium constant K and β

http://www.youtube.com/watch?v=LydEaN8-WJ8

Structural effect

Factors affecting stability of complexes

Example 1

Assume that in the reaction of Cu2+ with ammonia, the only complex ion to form is the tetra-ammine species, [Cu(NH3)4]2+. Given a solution where the initial [Cu2+] is 0.10M, and the initial [NH3] is 1.0M and that β4 = 2.1 x 1013, calculate the equilibrium concentration of the Cu2+ ion.

http://wwwchem.uwimona.edu.jm/courses/IC10K1.html

Examples Calculate the equilibrium concentration of the Fe3+ ion in a solution that is initially 0.10 M Fe3+ and 1.0 M SCN-, given that β2 for Fe(SCN)2

+ = 2.3 x 103

(1) Irving-Williams series

M-L bonds become more covalent

Hydration energy gets higher if LFSE is more negative !

(2) Ligand Field Stabilization Energy

(3) Jahn-Teller Effect

Estimate which d-orbital occupation(s) can cause a JT effect ….. (distinguish between high- and low-spin)

https://www.youtube.com/watch?v=Omy3NTu7pf8

Example Cr(II)(H2O)6

Why is a distortion to D4h preferred over regular Oh ?

Solution: estimate the LFSE for both symmetries For both symmetries, the 3 electrons in the lower levels have LFSE = 3 (-3/5 ∆o) But the one electron in the upper level is lower in energy for D4h therefore the total LFSE is more negative (more stable)

(4) HSAB principle

• Hard Lewis acids: • Atomic centres of small ionic radius • High positive charge • Species do not contain electron pairs in their valence shells • High energy LUMO • Soft Lewis acids: • Large radius • Low or partial δ+ positive charge • Electron pairs in their valence shells • Easy to polarise • Low energy LUMOs, but large magnitude LUMO coefficients • Hard Lewis bases: • Small, highly solvated, electronegative atomic centres: 3.0-4.0 • Species are weakly polarisable • High energy HOMO • Soft Lewis bases: • Large atoms of intermediate electronegativity: 2.5-3.0 • Easy to polarise • Low energy HOMOs but large magnitude HOMO coefficients • Borderline species have intermediate properties.

http://www.meta-synthesis.com/webbook/43_hsab/HSAB.html

“Hard acids” Small metal ions with high charge (Fe 3+, Co 3+, Ni 3+)

“Soft acids” Bigger metal ions with low charge (Cu 2+, Ag +, Au +, Pt 2+)

“Hard bases” ligands with low polarization (F -, OH -, Cl -, NH3) “Soft bases” ligands with high electron density (I -, SCN -, CN -, CO, PR3)

“basicity” high electron density

(5) Basicity of Ligands

Estimate basicity of molecules

Order these ligands from lowest to highest basicity NH3 PH3 P(CH3)3 SR2 F- Br- OH- CN-

Main effect is the electron density ! Determined by: electronegativity of atom with lone pair and/or electron-pull or donate effect of neighbor atoms

(7) Chelate effect

https://www.youtube.com/watch?v=iiR75lB9PKo

Example:

Example: the driving force is the increase in entropy

Conclusion

Complexes formed by multidentate ligands are much more stable than those formed by “normal” ligands !

(8) Bulk and Size of Ligands

(9) Macrocyclic effect

Review Questions

Review Questions

(1) Explain why:

(2) Explain why:

(3)

(4) see explanation below !

Orbit Angular Momentum Contribution to Magnetism

In order for an electron to have orbital angular momentum, it must be possible to transform the orbital it occupies into an entirely equivalent and degenerate orbital by rotation. The electron is then effectively rotating about the axis used for the rotation of the orbital. In an octahedral complex, for example, the three t2g orbitals can be interconverted by rotations through 90 deg; thus, an electron in a t2g orbital has orbital angular momentum. The eg orbitals, having different shapes, cannot be interconverted and so electrons in eg orbitals never have angular momentum. (The equivalent rotated orbital must NOT have already a single electron, so in t2g (3) there is NO orbital contribution to m !)

(5)

(6)

(2)

(1)

(3)

(4)

(6)

(5*)

* 1 / 3 / 4 / 5 / 0 / 3 / 2 unpaired el.

(7)

(8)