Here it is
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Diborane is Lewis acid and ammonia or amines are Lewis bases.Diborane electron deficient and ammonia is electron rich. If the ligand amine is bulky then boron cannot accommodate both of them on it and in that case there is symmetrical cleavage due to bulkiness of the amine group. However if ligands are less bulky like ammonia then both the ligands can go on the same boron which results in unsymmetrical cleavage.

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