This Will hopefully explain 3c-4e bonding in HF2-, Al2Cl6, solid BeCl2 etc.
HF2- has 3c-4e strongest symmetric H-bonding
AlCl3 in vapor phase or in non-polar solvent is dimeric Al2Cl6 hence sp3 hybridised having 3c-4e bonds
In transition state of SN2, carbon is pentavalent but sp2 hybridised having 3c-4e bonds
BeCl2(g) is sp hybridised, linear molecule, like HgCl2
whereas Solid BeCl2 is polymeric and sp3 hybridised having 3c-4e bonds.
BeH2(g) again is sp hybridised, linear
But polymeric BeH2(s) is has 3c-2e bonds just like in B2H6 because central Be-H-Be bridged bond can involve only two electrons.
Errata!! in video, its GaH4Cl2 rather than GaH2Cl2