π-System Lewis acids are reduced by s-HOMO Lewis bases:
Hydride ion donor reagents, such as lithium aluminium hydride or sodium borohydride, both Type 6 complexes:
H2 plus transition metal catalyst:
During such reductions the hydrogen is likely to be in the form of a transition metal/hydride Type 21 complex.
The saline hydride reagents, NaH etc., Type 5 complexes, are generally too Brønsted basic to be used as organic reducing agents.