This is a (2x2) structure

Rationale

Once again it should be noted that the unit cell must be a repeating unit capable of generating the whole surface structure : in this case the structure cannot be described as a c(2x2) or (2x2)R45 structure since the molecule in the centre of any defined centred cell would be crystallographically distinct from those at the corners (since it would be tilting in the opposite direction). The overlayer shown strictly has a primitive (2x2) unit cell containing two CO molecules.