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Chemistry - complex ions

i am a little confused - how do you know when a complex has 4 or 6 ligands because apparently it is different with NH3 from H2O .... :confused:

Reply 1

kriztinae

14

noggin

i am a little confused - how do you know when a complex has 4 or 6 ligands because apparently it is different with NH3 from H2O .... :confused:

hmm yea i get confused there too? doesnt it depend on if its full ligand exchange or something?or am i totally wrong?

Reply 2

misty

2

hmm.... look here http://www.s-cool.co.uk/topic_quicklearn.asp?loc=ql&topic_id=14&quicklearn_id=7&subject_id=7&ebt=160&ebn=&ebs=&ebl=&elc=13

not totally sure myself, need to revise chem before thursday lol

Reply 3

noggin

OP

7

ugh i had forgotten about the colours of different ones as well - argh! i really really hope it doesnt come up! its in unifying concepts, right?

Reply 4

minkailin

i think the basic principle is that it depends on the ligand's size, eg. Cl- is large so usually only 4 Cl- are bonded to the central metal cation.

Reply 5

noggin

OP

7

minkailin

i think the basic principle is that it depends on the ligand's size, eg. Cl- is large so usually only 4 Cl- are bonded to the central metal cation.

ta, any more digestable bits of info? i find reading big blurbs about them just goes into one eye and out the other.

Reply 6

boygenious

noggin

i am a little confused - how do you know when a complex has 4 or 6 ligands because apparently it is different with NH3 from H2O .... :confused:

I would assume all transition metals have octahedral shapes when ammonia or water are the ligands. However with copper, it doesn't fully substitute with excess ammonia and instead you get [Cu(NH3)4(H2O)]2+. Look up the John-Teller effect if you want an explanation regarding this. Chloride ions form tetrahedral complexes as they are charged and quite big.

Reply 7

Ralfskini

10

The vast majority of transition metal ions in solution have six water ligands (there are a few notable exceptions although it is not necessary to learn these). In reactions with bases, the number if protons lost and hence the number of hydroxyl groups surrounding the precipitate depends on the charge of the ion.

Reply 8

hornblower

6

noggin

i am a little confused - how do you know when a complex has 4 or 6 ligands because apparently it is different with NH3 from H2O .... :confused:

You don't need to know about it at A-level. Do whatever you have learned - the examiner will give you the marks whether you have six or four with two waters. However, just go for six everytime - it's easier.

Reply 9

spali

all small neutral ions =octahedral
all large ions =tetrahedral (as they are too large to fit round)
most anions will be tetrahedral as unlike the neutral ions they repel each other e.g [Al(OH)4]-
but then u have to remember the atypical, eg chromium forms octahedral with OH-
and complexes like Ag ones are always linear
and so are Cu+ complexes as these have a full outer electron shell (so this doesnt have a colour either)
hope thats clearer now!!

what exam board are u doing coz u always have to draw ligands and complexes in AQA

Reply 10

noggin

OP

7

spali

what exam board are u doing coz u always have to draw ligands and complexes in AQA

OCR - maybe i wont have to draw them, eek hope not anyway! i think that will just have to be a couple of marks i decide to lose :eek:

Reply 11

Flicker

13

noggin

OCR - maybe i wont have to draw them, eek hope not anyway! i think that will just have to be a couple of marks i decide to lose :eek:

if you look in chemistry 2 it tells you what it will be - [Cu(H2O)6] and [Cu(NH3)4(H2O)2]...it just is that as far as ocr are concerned, you're not going to get asked any other apart from them, just learn those ones, there's not many!

Reply 12

noggin

OP

7

allisandro

if you look in chemistry 2 it tells you what it will be - [Cu(H2O)6] and [Cu(NH3)4(H2O)2]...it just is that as far as ocr are concerned, you're not going to get asked any other apart from them, just learn those ones, there's not many!

wahey! thank you v. much! do you happen to know what colours those ones are?

Reply 13

ravs

heres are the colours of ions when added to ammonia or NaOH dropwise:

Cr-green ppt
Ni- green ppt
Fe2+= green/yellow ppt
Fe3+-red/brown ppt
Cu- Blue ppt
Co-Blue ppt
Mn2+-buff /beige ppt
Zn-white ppt

with excess NaOH, they all remain insoluble apart from Cr which forms green soln and Zn forms clear soln

with excess NH3 they all remain insoluble apart from Ni and Cu which both give blue solns and Zn again forms clear soln

but remember, Zn is not a T metal, and for this very reason

Reply 14

Flicker

13

noggin

wahey! thank you v. much! do you happen to know what colours those ones are?

[Cu(H20)6]2+ is royal blue...add NH3 it goes pale blue (Cu(OH)2 is formed) and then adding more makes it go deep blue, forming [Cu(NH3)4(H2O)2]2+.

if you add HCl to [Cu(H20)6] you make a yellow solution of [CuCl4]2-.

also you need to know how to make casualty blood! [Fe(H2O)6]3+ + SCN- (thiocyanate ions) = [Fe(H20)5SCN]2+ + H20

That's all the ligand substitution and complex reactions you need to know!

Reply 15

ravs

not on edexcel syllabus u dont, u literally just need to know wat i typed, but i assume ur talking about a diff syllabus

Reply 16

Flicker

13

ravs

not on edexcel syllabus u dont, u literally just need to know wat i typed, but i assume ur talking about a diff syllabus

yup she said she did OCR.

Reply 17

Emzeebubs

hey, i'm doing OCR chemistry too.....but ligands and complex ions etc are NOT in unifying concepts, they're in trends and patterns (just looked at my syllabus cus u scared me lol)

Reply 18

noggin

OP

7

Emzeebubs

hey, i'm doing OCR chemistry too.....but ligands and complex ions etc are NOT in unifying concepts, they're in trends and patterns (just looked at my syllabus cus u scared me lol)

wahey! i wondered why they hadnt come up in any of the unifying concepts past papers!! phew that gives me a few more days to try and understand them!

Chemistry - complex ions

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