[jayson]

In one of my organic chemistry textbook they list several compounds with central sulfurs with three attached ligands (=O and two other -R groups). They name these with R or S stereochemistry. I'm quite confused as with carbon the terms re and si would be used. I'm not really sure how to assign stereochemistry for these as there are only three groups. If I treat the lowest priority group as I woudl treat a hydrogen in a carbon center that leaves two groups that could be counted both clockwise and counterclockwise, suggesting, AFAIK, that R and S could not apply in this case. Any idea how I would go about assigning R and S to these centers?

Thanks in advance!
Jayson