georgeElsworth
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#1
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When you react ammonia with Cu^2+ you get Cu(NH3)4^2+

now why do four ammonia molecules bind to the copper ion, i have looked at the electron configuration and electrons feel all shells and leave a 5d5 shell. Why do 4 ammonia double bonds bind to the copper ion, gr
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Wenzel
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I'm fairly certain they're dative covalent bonds, not double bonds.

[Cu(H2O)6)]2++ 4NH3 ---> [Cu(H2O)2)(NH3)4]2+ + 4H2O

You just get 4 of the water molecules replaced with ammonia (if in excess, other wise you end up with [Cu(H2O)4)(OH)2]). Ammonia has a free lone pair of electrons, which is why it acts as a ligand (acts as a base when not in excess) here.
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_Aladdin
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#3
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u can get 6 ammonias but you have to use a greater conc of ammonia, and not acqeuous ammonia, has to be liquid. and they are dative bond not double bonds.
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zzzzzoe
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i thought that you only got the H20 and NH3 compound if you use ammonia in excess?
and, dropwise you got [Cu(H20)4(OH)2]
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xxlawyergirlxx
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woop I'm revising this right now for my chemistry exam on thursday
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Wenzel
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(Original post by zzzzzoe)
i thought that you only got the H20 and NH3 compound if you use ammonia in excess?
and, dropwise you got [Cu(H20)4(OH)2]
That's what I said.:p:

Not in excess ammonia: [Cu(H2O)6)]2++ 2NH3 ----> [Cu(H2O)4)(OH)2] + 2NH4+ (ammonia acting as a base)
In excess ammonia:
[Cu(H2O)6)]2++ 4NH3 ---> [Cu(H2O)2)(NH3)4]2+ + 4H2O (ammonia acting as a ligand/lewis base)
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zzzzzoe
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yeah! thats what i meant too
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-Kav-
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(Original post by Wenzel)
I'm fairly certain they're dative covalent bonds, not double bonds.

[Cu(H2O)6)]2++ 4NH3 ---> [Cu(H2O)2)(NH3)4]2+ + 4H2O

You just get 4 of the water molecules replaced with ammonia (if in excess, other wise you end up with [Cu(H2O)4)(OH)2]). Ammonia has a free lone pair of electrons, which is why it acts as a ligand (acts as a base when not in excess) here.
He's quite right, though, in that square planar tetraamminecopper(II) can be formed; one would assume under anhydrous conditions. As to why, that's pretty difficult to explain without going into the intricacies of crystal field theory. Comparing stabilisation energy values for hexaamminecopper(II) and tetraamminecopper(II), you might well find that the latter is preferred. Otherwise, copper(II) is pretty small, so steric hindrance might play a part.
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georgeElsworth
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sorry i did mean dative, i didnt actually think they were double lol. I understand that it happens but thwn looking at the electro config of Cu^2+ it is clear that it is not 8 electrons short of completing its outer shell...so why does this happen
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-Kav-
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(Original post by georgeElsworth)
sorry i did mean dative, i didnt actually think they were double lol. I understand that it happens but thwn looking at the electro config of Cu^2+ it is clear that it is not 8 electrons short of completing its outer shell...so why does this happen
As I say, all those little guidelines about octets and complete shells are just that, guidelines. Coordinate chemistry in particular pays scant regard to them due to energy differences in the 'frontier' orbitals of the complex. Ultimately, if you can lower the overall energy of the orbitals that your electrons are occupying, you can form your bonds. This often coincides with filled shells, but it doesn't have to. If you want a more detailed explanation, I'll be happy to give it, but it might go on at some length. :p:
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Wenzel
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(Original post by georgeElsworth)
sorry i did mean dative, i didnt actually think they were double lol. I understand that it happens but thwn looking at the electro config of Cu^2+ it is clear that it is not 8 electrons short of completing its outer shell...so why does this happen
The electron configuration of copper is [Ar] 3d10 4s1. When copper becomes 2+ it loses the 4s electron and and one of the 3d electrons (so [Ar] 3d9), which leaves the 4s level empty. The 4s/4p/4d shells all get rearranged so that they're on the same energy level. When the ligands bond, they fill the empty 4s, 4p and two of the 4d sub-shells. The reason there's 4 ammonia and 2 water is because that's just the maximum amount that can fit around the copper atom due to their size.
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-Kav-
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(Original post by Wenzel)
The electron configuration of copper is [Ar] 3d10 4s1. When copper becomes 2+ it loses the 4s electron and and one of the 3d electrons (so [Ar] 3d9), which leaves the 4s level empty. The 4s/4p/4d shells all get rearranged so that they're on the same energy level. When the ligands bond, they fill the empty 4s, 4p and two of the 4d sub-shells. The reason there's 4 ammonia and 2 water is because that's just the maximum amount that can fit around the copper atom due to their size.
Is this really what they're teaching at A-level? It looks completely mad, to my mind. For starters, you're leaving a gap at 3d in favour of filling a 4d, which is very odd. You certainly can't apply hybridisation arguments to complexation, because it would ruin explanations of paramagnetism, colour, stability, everything. If you're going to involve your ligand electrons in your description of energy levels, you're going to have to apply ligand field theory, and the resulting orbitals are by no means at the same energy level. Most of the explanations of coordinate chemistry are based entirely on them not being degenerate. Sorry if this comes off as a bit of a rant, but I despair of the A-level syllabus sometimes.
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Wenzel
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(Original post by -Kav-)
Is this really what they're teaching at A-level? It looks completely mad, to my mind. For starters, you're leaving a gap at 3d in favour of filling a 4d, which is very odd. You certainly can't apply hybridisation arguments to complexation, because it would ruin explanations of paramagnetism, colour, stability, everything. If you're going to involve your ligand electrons in your description of energy levels, you're going to have to apply ligand field theory, and the resulting orbitals are by no means at the same energy level. Most of the explanations of coordinate chemistry are based entirely on them not being degenerate. Sorry if this comes off as a bit of a rant, but I despair of the A-level syllabus sometimes.
The reason why and how they bond isn't on the syllabus, that's just what my teacher taught our class (and he simplifies everything that isn't on the syllabus). All we need to know is that they form dative covalent bonds, their colours and the reactions involved.

I guess I should have added it's just how our teacher taught us it. Sorry.
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-Kav-
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(Original post by Wenzel)
The reason why and how they bond isn't on the syllabus, that's just what my teacher taught our class (and he simplifies everything that isn't on the syllabus). All we need to know is that they form dative covalent bonds, their colours and the reactions involved.

I guess I should have added it's just how our teacher taught us it. Sorry.
I'm sorry myself. Shouldn't be taking out my anger over a government-implemented syllabus on a guy who's just trying to help people. I think being away from university has made me a bit over-passionate about chemistry (not something I thought I'd be saying a few weeks ago.)
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cpchem
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(Original post by -Kav-)
Is this really what they're teaching at A-level? It looks completely mad, to my mind. For starters, you're leaving a gap at 3d in favour of filling a 4d, which is very odd. You certainly can't apply hybridisation arguments to complexation, because it would ruin explanations of paramagnetism, colour, stability, everything. If you're going to involve your ligand electrons in your description of energy levels, you're going to have to apply ligand field theory, and the resulting orbitals are by no means at the same energy level. Most of the explanations of coordinate chemistry are based entirely on them not being degenerate. Sorry if this comes off as a bit of a rant, but I despair of the A-level syllabus sometimes.
:ditto:

Good old LFT, LFSE, the spectrochemical series - all that crap. Coordination chem is your friend - and mine!
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charco
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(Original post by -Kav-)
Is this really what they're teaching at A-level? It looks completely mad, to my mind. For starters, you're leaving a gap at 3d in favour of filling a 4d, which is very odd. You certainly can't apply hybridisation arguments to complexation, because it would ruin explanations of paramagnetism, colour, stability, everything. If you're going to involve your ligand electrons in your description of energy levels, you're going to have to apply ligand field theory, and the resulting orbitals are by no means at the same energy level. Most of the explanations of coordinate chemistry are based entirely on them not being degenerate. Sorry if this comes off as a bit of a rant, but I despair of the A-level syllabus sometimes.
Theories are ideas that allow us to model a specific situation, using terminology and concepts appropriate to the level applied. In terms of transition metal chemistry we accept that the donated electrons from the incoming ligands must enter some adequate orbital. It is also apparent that the 3d orbitals are not used in this bonding as the magnetic properties, colour etc demonstrate.
Therefore it is taught that there is some kind of hybridisation involving the 4s, 4p and 4d orbitals to form an octahedral set of bonding orbitals that can accept the lone pairs of the ligands.
This seems reasonable as the SF6 molecule can also be rationalised in a similar way (only using hybridised singly occupied degenerate orbitals).
It is also an explanation that fits the level of chemistry being taught and allows modelling according to the accepted theories.
Logically such theories are bound to be aproximations as were the octet idea and LCAO before them.
Students cannot be presented with group theory, quantum mechanics and molecular orbital concepts without a foundation. There are no lies involved, merely simplifications based in the historical development of the subject.
I have little doubt that in the (not too) distant future all matter will be described in terms of energy extrusions from a parallel universe, or something equally bizarre, rendering quantum mechanics obsolete, but still taught in the realm of the 'cerebritos'.
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Excalibur
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While on a similar subject - is the hydrated Zn2+ a [Zn(H2O)4]2+ or [Zn(H2O)6]2+ molecule? Different books tell me differently...
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Wenzel
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(Original post by Excalibur)
While on a similar subject - is the hydrated Zn2+ a [Zn(H2O)4]2+ or [Zn(H2O)6]2+ molecule? Different books tell me differently...
The latter ([Zn(H2O)6]2+) and it's colourless.

And since we're talking about zinc, with excess ammonia is it [Zn(H2O)2(NH3)4]2+ or [Zn(NH3)4]2+?
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-Kav-
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(Original post by charco)
Theories are ideas that allow us to model a specific situation, using terminology and concepts appropriate to the level applied. In terms of transition metal chemistry we accept that the donated electrons from the incoming ligands must enter some adequate orbital. It is also apparent that the 3d orbitals are not used in this bonding as the magnetic properties, colour etc demonstrate.
Therefore it is taught that there is some kind of hybridisation involving the 4s, 4p and 4d orbitals to form an octahedral set of bonding orbitals that can accept the lone pairs of the ligands.
This seems reasonable as the SF6 molecule can also be rationalised in a similar way (only using hybridised singly occupied degenerate orbitals).
It is also an explanation that fits the level of chemistry being taught and allows modelling according to the accepted theories.
Logically such theories are bound to be aproximations as were the octet idea and LCAO before them.
Students cannot be presented with group theory, quantum mechanics and molecular orbital concepts without a foundation. There are no lies involved, merely simplifications based in the historical development of the subject.
I have little doubt that in the (not too) distant future all matter will be described in terms of energy extrusions from a parallel universe, or something equally bizarre, rendering quantum mechanics obsolete, but still taught in the realm of the 'cerebritos'.
I appreciate that simplified models have to be used in teaching and in prediction, of course I do, but I don't buy it in this case. A hybridisation model of coordinate complexes is all but useless, as it explains nothing but the geometry. Degenerate orbitals would make coloured complexes impossible, for one thing, and I contest that A-level students time would be much better spent learning the fundamentals of Crystal Field Theory (for which they already have most of the 'tools') rather than learning an enormous list of complex colours without any explanation as to how they arise. Hybridisation is a very useful model for organic chemistry and, to some extent, for aspects of main group chemistry, but I can see no excuse for applying it to co-ordinate chemistry.
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EierVonSatan
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(Original post by -Kav-)
I appreciate that simplified models have to be used in teaching and in prediction, of course I do, but I don't buy it in this case. A hybridisation model of coordinate complexes is all but useless, as it explains nothing but the geometry. Degenerate orbitals would make coloured complexes impossible, for one thing, and I contest that A-level students time would be much better spent learning the fundamentals of Crystal Field Theory (for which they already have most of the 'tools') rather than learning an enormous list of complex colours without any explanation as to how they arise. Hybridisation is a very useful model for organic chemistry and, to some extent, for aspects of main group chemistry, but I can see no excuse for applying it to co-ordinate chemistry.
I'm afraid that i have to agree with charco, crystal field theory has never been an A-level topic and frankly shouldn't be. Hybrid models sp2d (square planar), sp3d (triginal bipyramidal), sp3d2 (octohedral) hybrids are reasonable approximations, even though yes i know there are many exceptions - this is why MO theory has largely replaced it. Its too much to ask A-level students to cope with non-bonding and antibonding orbitals, symmetry mixing and the variation principle etc
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