Chem HL Help

[Jarz]

Hey guys,

So I got most of chem hl down, however I can not seem to understand the following two things, what are ligands, how to I calculate things like their strength etc. and what are emission spectrum? I get the general idea but I don't seem to get it completely, as when there are questions I get really confused. 

If anyone has any links or can help me please tell me! Thanks a lot!

[Jarz]

For example I understand that they make electrons jump between D-subshells but I dont get what that does... <-- if that is correct haha

[kw0573]

Have you looked at youtube videos by MSJChem (Mike Sugiyama Jones), Richard Thornley, or Andrew Weng? They follow the IB syllabus and provide videos for quick review.

So the main idea is that ligands can split the transition metal's 5 degenerate (same-energy) valence d orbitals into different energies, depending on the transition metal ion, geometry of the complex and the specific ligand(s). In weak field ligands, the resulting d orbitals are still close in energy (small bandgap), such that they remain essentially degenerate. In strong field ligands, the activation energy to go from lower d orbitals to higher d orbitals are large enough such that electrons fill the lower d orbitals first before the higher ones. In the common octahedral geometry (ML6), the splitting results in 2 d orbitals higher in energy than the other 3. Other complexes have different splitting patterns.

The colour of the complex depends on the light that is most absorbed. This energy will partially excite an electron from a lower d orbital to a higher one and partially goes to thermal energy. The equation E = hν relates the photon energy E to the frequency of the peak absorbance light v, or E = hc/λ, with c the speed of light and λ the wavelength. The colour of the complex is then the light that passes through (or get transmitted), which is determined using the colour wheel. So if using E = hv you calculate 700 nm (v = 4.3× 10^17 Hz) or red light, then the complex colour is green because red gets absorbed so green passes through.