Non-heme iron complexes with cis-Fe-III(OH)(SAr/OAr) coordination were isolated and examined for their reactivity with a tertiary carbon radical. The sulfur-ligated complex shows a temperature dependence on (OH)-O-center dot versus ArS center dot transfer, whereas the oxygen-ligated complex does not. These results provide the first working model for C-S bond formation in isopenicillin N synthase and indicate that kinetic control may be a key factor in the selectivity of non-heme iron "rebound" processes.