Boron exhibits diagonal relationship with

Boron forms acidic oxide similar to silicon, unlike aluminum which forms amphoteric oxide. That’s why boron hydroxide is acidic whereas aluminum hydroxide is amphoteric.
Boron and silicon form many polymeric ions such as borates and silicates respectively by sharing oxygen atoms whereas aluminum forms no such polymeric ions.
Hydrides of boron and silicon are gases whereas aluminum hydrides are polymeric solids. Hydrolysis of boron and silicon hydrides is fast and they are highly inflammable.
Boron shows some relationship with silicon which is nothing but a diagonal relationship.