[041087]

Hi! I'm doing Edexcel Chem 5 in few weeks. Just have a question with one of the past years (Jan 2012 Q21b(v))
Explain, in terms of its structure, how H2NCH2CH2NH2 can act as a bidentate
ligand whereas H2NNH2 cannot.

The marking as following
Any two from
Both have two nitrogen atoms with lone pairs or implied (1)
or
Far enough apart/longer chain in between in en (but not in
hydrazine)/too close in hydrazine/hydrazine is too short/not as long (1)
or
Dative bonds/lone pairs too close/repel in hydrazine (1)
OR
for two marks
Forms 5-membered ring (with en with no angle strain/stable) (2)
or
Bond angles too acute/too much ring strain in hydrazine (2)

Actually i dun really understand the marking
Can somebody help?

[EierVonSatan]

It's saying that the carbon chain in ethylenediamine allows it to form a ring whist hydrazine is too short to do the same. Is that what you're confused about?

[Billton]

A bidentate ligand can donate 2 pairs of electrons. Ethylenediammine is probably the most common one, and it differs from hydrazine because en can "wrap" itself around and form coordination bonds at 2 points

[041087]

Actually what is the meaning of Hydrazine?
Also how the distance affect the ligand form?


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[charco]

(Original post by 041087)
Actually what is the meaning of Hydrazine?
Also how the distance affect the ligand form?


This was posted from The Student Room's iPhone/iPad App

If hydrazine attached itself to a metal ion at two points using its two nitrogen lone pairs it would produce a ring structure with only three sides. This would be very strained as it would have to have an internal angle of 60º. Consequently it can't form.

However, the same situation with ethylenediamine forms a ring with five sides and an internal angle of 108º. This is perfectly stable and unstrained.

[EierVonSatan]

(Original post by 041087)
Actually what is the meaning of Hydrazine?
Also how the distance affect the ligand form?

Hydrazine is the name for H2N-NH2

Your second question isn't clear, but if you're asking why does the length matter - then consider two pieces of string, one is 1cm long the other is 5cm. The longer one is easier to make a ring out of (it's a crude analogy but it works)

This was posted from The Student Room's iPhone/iPad App

This wasn't.

[041087]

(Original post by EierVonSatan)
Hydrazine is the name for H2N-NH2

Your second question isn't clear, but if you're asking why does the length matter - then consider two pieces of string, one is 1cm long the other is 5cm. The longer one is easier to make a ring out of (it's a crude analogy but it works)



This wasn't.

One more question, are the naming of ligand is in our scope of the examination?

[charco]

(Original post by 041087)
One more question, are the naming of ligand is in our scope of the examination?

You can read the edexcel spec here