Single-atom　catalysis　is　recognized　as　a　frontier　of　heterogeneous　catalysis　for　its　efficient　utilization　of　metals　and　the　possibility　to　engender　unusual　reactivity.　Yet,　despite　the　observation　of　single　atoms,　understanding　their　coordination　structures　and　developing　structure–property　relationships　remains　challenging　due　to　the　structural　complexity　of　support　surfaces.　Here,　using　single-crystalline　MgO(111)　two-dimensional　nanosheets　and　a　surface　organometallic　chemistry　method,　we　describe　the　formation　of　highly　dispersed　Ir(III)　sites　(isolated　at　0.1　wt%,　and　Ir　pairs　and　trimers　at　1　wt%)　with　well-defined　coordination　structures.　These　species　display　unique　catalytic　properties　in　the　coupling　reaction　of　benzene　and　ethylene　to　form　styrene,　a　reactivity　that　contrasts　with　conventional　homogeneous　and　heterogeneous　iridium　catalysts　that　yield　ethylbenzene.　The　similar　activities　for　high-　and　low-loading　catalysts　suggest　that　iridium　sites,　whether　isolated　or　in　the　form　of　clusters　(for　example　Ir3),　have　similar　activity,　consistent　with　the　involvement　of　surface　dynamics.　[Figure　not　available:　see　fulltext.]　©　2021,　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Limited.