Pt(111)+(disordered)-C2H4
Pt(111)+(2x2)-C2H3

Intact "di-sigma" ethylene (C2H4, shown at left),
is disordered at any temperature and coverage.
Diffuse LEED analysis at 200 K finds its adsorption to be asymmetrical:
one C bridge-bonded, the other near-top bonded.

Ethylidyne (C2H3, shown at right), results from ethylene dehydrogenation.
It is a very stable species, chemically largely unreactive.
It orders well in a (2x2) superlattice.
(Hydrogen positions are guessed.)