I imagine that you're talking about boron hydrides..
Basically when these were discovered the no. of electrons were found to be insufficient to fulfil the prevalent theories of bonding...
...So the theoreticians came up with the idea of banana bonds where the hydrogens act as a kind of bridge between two boron atom centres (hence 2e3c bonds) or three boron atoms bonded together.
Chemistry has evolved theories to fit experimental data since time began and all the theory of banana bonds attempts to do is explain how electron deficient borohydrides can possible bond together.
How do you predict them?
By counting the available electrons and the number of theoretical bonds. If there cannot be two electrons per bond according to the structure then some other form of bonding must take place.
Look up boron hydrides eg borane, diborane nido-boranes, etc.
But this isn't High School chemistry?