importance of models - ligands

[lepton_index]

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Syllabus point: discuss the importance of models in developing an understanding of the nature of ligands and chelated ligands, using specific examples

I don't really understand the meaning of the word "model" in this question. Does it imply models like the Valence bond theory, the crystal field theory,... or physical 3D models?

[tomorrows_angel]

there are three models that we have to know that i'm aware of:
lewis acids
crystal field theory
valence bond theory...
i don't think it means physical models.

[nit]

Hmmm, we didn't even look at crystal field theory in class...oh well it's easy enough i guess

[S_Kaye]

We did it as physical models.
We talked about Molymod Kits
And Lewis Dot Diagrams.
And then just discussed pros and cons to do with ligands, bonding etc

[mitochondria]

Those points Kaye and angel mentioned are right. Just a note for angel: A model includes both physical models (such as molecular models) and theorie. Models are developed based on experiment and observation over time and a good model helps us to predict the behaviour of similar phenomena.

Kaye: Molymod is a brand name

[lepton_index]

Thanks. I think physical models are more likely, since theoretical models are quite complicated.

[jang]

specific examples --> that is of models yeh? not specific ligands/chelated ligands?

umm then y does the textbook - chem contexts - have all that info about haemoglobin and placitin or watever etc?

[lepton_index]

I think we need to list out specifically HOW the models help to develop an understand of ligands and chelated ligands (e.g.: help in identifying the bonding site). Then, we need to give specific examples of ligands and chelated ligands (e.g.: EDTA), then identify the bonding site in there.

[randhi]

i'm so sorry guys but what how do u connect lewis dot diagrams and valence bond theory into it?? i get the chelated ligands one finally!! can any one explain to the studpid one please...

[cameron0110]

Dot diagrams are used to show the coordinate covalent bond diagrammatically- so in reality they are the easiest because they show the ligand bonding to the metal. The valence bond discusses the way electrons are donated when the ligand and the metal hybridise and allow for the prediction of magnetic and geometric properties of the complex and therefore allow for an understanding of the ligands and chelating ligands forming complex ions. There is undoubtedly more I've missed if anyone else wants to throw something in.

[nit]

Cameron, I don't think the VB model allows you to predict magnetisation. VB is good for the structures, but the problem is, it's far too complicated and ungainly to deal effectively with - ie its just not general enough...(just an example: sp3d hybridisation - trigonal bipyramidal structure - can also turn out ot be dsp3 depending on the metal and the available orbitals, and it gets a lot worse from there. It's difficult to form a consistent model in this way that is meaningful)
Magnetisation is best observed through Crystal Field Theory, in which ligands are approximated by negative point charges, and you observe the shifts this causes to the d orbitals of the metal (formation of t2g and eg subsets for an octahedral site), which in turn, depending on the nature of the ligand, gives you info abt the number of unpaired electrons in the ligand and hence whether it is para- or diamagnetic. Furthermore the energy differences produced as a result of the d-orbital splitting give you the colours or lack of them.
I'm not entirely sure what the dot-point wants, but I think it's fairly safe to talk firstly about the simplest of all models- the ball and stick - followed by lewis dot diagrams/lewis acid base theory, then VB and the basics of crystal field theory...no doubt Lewis would be the focus of analysis. For each of these you would assess the positives and the negatives and come to a conclusion I'd say.

[cameron0110]

The magnetism in VB comes from whether there is forced pairing or not. In a d2sp3 hybridisation there is forced pairing leading to it being diamagnetic. In sp3d2 there is no forced pairing which makes the complex paramagnetic because there are still unpaired electrons that can respond to a magnetic field.

[nit]

It is QMOT, which superceded the valence bond model that explains paramagnetism. VB theory can not determine accurately whether a substance is paramagnetic...liquid oxygen is the best example for this. Certainly, there are instances in which simply listing the d-orbitals and s and p orbitals will determine magneticity, but in general, this is not recommended - its easy to go wrong in predicting the no. of unpaired electrons on a metal ion and consequently the "spin only" magnetic numbers will be stuffed up. For the complete picture at an accessible level, QMOT and CFT are the way to go.