[Oblivious2Kez]

Hi Guys/Gals

I've got a Homework problem for a co-ordination chemistry paper that i'm having trouble with:

TPP-BH₃ ( Borane Triphenylphosphine) Lewis acid/Base adduct can be made by reacting BH₃ with TPP. The TPP can be regenerated from TPP-BH₃ upon reaction with amines such as diethylamine, morpholine, pyrrolidine... etc Why does this work?.

My thought process and correct me if i'm wrong, is that the B in the B-P bond is delta positive (electropositive) and that that bond is quite weak due to the large steric hinderence of the Phenyl groups on the P atom. Therefore the lone pair of electrons on the Nitrogen can attack the boron and the electrons that were in the P-B bond can transfer onto the P atom thus giving back TPP. However when I look at further questions in the homework the structure of the TPP-BH₃ complex has a - sign on the B atom and + on the P atom. which confuses me.

Any Help would be greatly appreciated

Cheers

[chenbeier]

The B-N  bond is stronger as the B-P bond. So an exchange from Phosphin to Amin is easily.

[Oblivious2Kez]

Thanks for your reply.

Could you possibly explain the reasoning behind why the B-N bond is stronger?

[wildfyr]

I think its because of atomic orbital overlap. Boron and nitrogen are in the same row and of similar size. The geometry of their bonding orbitals is more favorable. Boron and phosphorous are in different rows, and have worse orbital geometry.

Its' the same reason an acid fluoride is a stable compound whereas an acid bromide is highly susceptible to hydrolysis (oxygen is in the same row as carbon in this case, and is displacing bromine, which is in the third row).