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Why and how does Prussian Blue form

Dear Reader,

Welcome back and I have to warn you fine folk that I am still thinking about Prussian blue the wonder substance which helps us to manage the radioactive cesium from the Fukushima accident.

While on a boat crossing the north sea I asked myself the question of why does Prussian blue form and how. I think that I have come up with an answer. It is important for us to start with the unfriendly sounding molecule hydrogen cyanide. It goes backward and forwards. It is refined, very much maligned and misunderstood. Go easy on this fellow, he must never be abused. He gets the metals going and you find him fizzing in the corner in the bleach bin.

Some of you may have spotted the reference to 1980s culture, those of you who have not then do not worry. All will become clear soon. It is important to bear in mind that Prussian blue will not give you cyanide poisoning.

HCN is a very refined fellow, the modern and green way to make the dinitrile required for the production of the 1,6-diaminohexane required for nylon-6.6 production is to use hydrogen cyanide (with a nickel catalyst) rather than using sodium cyanide. So the next time some asks you to name a green reagent you can say “hydrogen cyanide” in a truthful way. While it is a toxic reagent it is more green than sodium cyanide as its use forms less toxic solid waste which is hard to deal with.

For a process to be truly green it must satisfy three things.

1. Be economically sustainable (Eg process for making aspirin at £ 10 per gram will not be OK)

2. Be environmentally sustainable, it must not guzzle resources or spew out vast amounts of waste for a small amount of product (Eg if I have to cut down a square mile of rainforest and kill five rare birds to make you an egg sandwich then this method is not an OK egg production system)

3. Be socially sustainable (Eg if a process requires small children to climb up chimneys then it will not be considered morally acceptable. As a result it will be impossible to sustain the process in today’s Soceity)

Next HCN is a very maligned and misunderstood substance; it is a toxic gas but if we want to base our vilification of gases on purely their toxicity then hydrogen sulphide beats it in the top ten worst ever gases. My own view is that carbon monoxide is more of a fright gas as CO has absolutely no smell and is much more common (check your when your gas appliances were last checked by a service engineer). But as a result of the fact that HCN was the poison gas used at some Nazi extermination camps, in the American gas chamber and in many detective stories hydrogen cyanide has acquired a super nasty reputation. It is interesting to note that carbon monoxide was also used by the Nazi murderers (the gas van), but why then has CO not become viewed with equal horror by the public ?

I would say that as a chemist or an industrial worker it is important to avoid breathing in or otherwise absorbing HCN, it is bad for your health. As well as the dire short term effects which are well known it can have some horrible long term effects which are sometimes seen in parts of Africa where people tend to live on a vegetable known as cassava. If you prepare this food wrongly then you will get a dose of cyanide in every meal, this can lead to chronic cyanide poisoning which causes among other things trouble with the nervous system. So my advice is to “go easy on your body” when working with cyanide. Do not abuse your body by forcing it to endure the stress of having to metabolize cyanide, take that bit of extra care to lower your occupational intake of cyanides.

The cyanide anion is a very strong ligand for many transition metals, indeed it does get the metals going. Sometimes in very much the wrong way, some time ago there was a large spill of cyanide waste in eastern Europe. It ended up in a river where it then killed the fish, one of the problems with cyanide it binds to an iron complex in mitochondria which then stops oxygen binding. As a result the fish could no longer use oxygen, as a result they died. But we need to understand why does cyanide bind to metals so well, the binding of cyanide to metals is much stronger than the binding of most simple monodentate ligands.

Monodentate ligands is a fancy term for a molecule or atom which binds through one atom onto a metal.

A snake which grabs you with its mouth is a monodentate animal

A crab which grabs you with both claws is a bidentate animal

A scorpion which grabs you with both claws and applies the stinger to you is a tridentate animal

The reason is the “backwards and forwards”, hydrogen cyanide when deprotonated forms the cyanide anion which uses a lone pair on the carbon to form a sigma bond to a metal. It also uses its empty pi* orbitals to suck electron density off of metals thus forming pi bonds to the metal.

Now we need to look at the orbitals of the hydrogen cyanide, the orbitals of the cyanide anion are almost identical.

Lets start with the HOMO, this is not a sexual term it means Highest Occupied Molecular Orbital in chemistry. Those of you who were expecting something sexual here, I am sorry but I am going to disappoint you, this blog is not about sexual matters. But feel free to carry on reading as you might find the chemistry interesting.

The HOMO of HCN

Here you should be able to see that on the nitrogen atom (blue atom) a lobe of the orbital pokes out into space away from the CH group, this part of the orbital will form the lone pair which allows the nitrogen to bind to things. Around the hydrogen atom is a big blue lobe. When the HCN loses a proton this will form a cloud of electron density which also pokes out into space. Here is another view which may make it more clear, the lone pair on the nitrogen and the blue blob on the carbon will allow it to form the sigma bonds which go to metal atoms.

Alternative view of HCN's HOMO

Here is a view of the HOMO of the cyanide anion, look at how similar it is to the HOMO of hydrogen cyanide.

Next here are two alternative views of the HOMO of the cyanide anion to allow you to have a better idea of the shapes of the orbitals.

The next thing to look at is the p orbitals of HCN, I have calculated these orbitals for the cyanide anion and they are the same shape so I will only show you one set. Here is one of the them.

One of the pi bonding orbitals of HCN

The hydrogen cyanide molecule has two occupied pi orbitals which look like a pair of sausages arranged parallel with the line between the carbon and the nitrogen. Here is a view of the other one.

A view of the other pi orbital

Next we have the pi* antibonding orbitals.

LUMO of HCN

HCN LUMO +1

I guess they looked the same to you, so here is the end view. Note that they are at ninety degrees to each other.

HCN LUMO

HCN LUMO +1

Now to understand antibonding, I want you to think of a nice person. How about St Francis of Assisi, after a wayward youth he grew up to be a man known for being kind to poor people and taking care of animals.

The anti-St Francis would be a nasty man who steals bread from staving single mothers and homeless men, for fun he throws
animals down the well.

The anti-St Francis is the total opposite of St-Francis, everything good about St-Francis has been turned into something horrible in the anti version. In the same way all the energy lowering effects of the bonding orbitals are turned into energy increasing effects in an antibonding orbital. Typically an antibonding orbital is more antibonding than the bonding orbital is bonding. So if you fill up both orbitals with electrons then overall the sum of the two orbitals is antibonding.

In case you want to see some of the other orbitals of HCN then here they are.

LUMO +3

LUMO +2

 

LUMO +1

LUMO

HOMO

HOMO -1

HOMO -2

HCN HOMO -3

HCN HOMO -4

I hope to bring you some more about our new friend (Prussian Blue) soon.

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