Crams chelate model

mycotheology

Heres a few nucleophilic additions:

I need to know in which of these reactions chelation will occur. In the first 2, there is no O, S or N atom to chelate the metal. I know that chelation will occur in reaction iii since the oxygen atoms will chelate the Mg²⁺ ion of the Grignard reagent. Its the next two that confuse me. Firstly, I know that Na⁺ ions are never involved in chelation but what about the boron atom? I know that boron atoms can be involved in chelation but I'm unsure in this case since the boron atom in the borohydride ion is not electron deficient like boron usually is. However, this reaction produces borane so will the borane produced participate in chelation in subsequent reaction?

As for the last reaction, I know that Ce is involved in chelation so the stereochemistry of that reaction is definitely dominated by chelation.