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Mechanism confusion for conversion of Mo(CO)6 to Mo(CO)5(pip)

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electrogeek:
Hi everyone,

I'm considering the following reaction:

Mo(CO)6 + pip  :rarrow: CO + Mo(CO)5(pip)

I think this is an example of a dissociative mechanism because an associative mechanism would lead to a 20-electron complex, making it unstable because it isn't in agreement with the 18-electron rule. But I'm a bit confused because if it is dissociative, then wouldn't Mo(CO)5 form an unstable intermediate because it would be a 16-electron complex?

I'm pretty sure it's dissociative because there's a similar example with W(CO)6 to form W(CO)5(PPh3), but wouldn't this lead to the same problem? I first thought that the solvent might temporarily interact with the intermediate to stabilise it before the pip ligand interacts, but the reaction is in toluene and I'm not sure if it can do η2 as well as η6 - I would say that it does because I know benzene can do η2, η4 and η6 bonding, but I'm not sure if the methyl group in toluene would change this.

Any help would be greatly appreciated! :)

Flatbutterfly:
If you are still looking for an answers to these question see Atwood's Book:

https://www.wiley.com/en-us/Inorganic+and+Organometallic+Reaction+Mechanisms%2C+2nd+Edition-p-9780471188971

Although the book is dated the chapter on substitution in metal carbonyls will have what you want.

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