mediator
intermediary /ˌɪntɚmˈiːdiːɚɹi/ /ˌɪntəmˈiːdiːəɹi/
go-between /ɡˌoʊbɪtwˈiːn/ /ɡˌəʊbɪtwˈiːn/
broker /ˈbroʊkər/ /ˈbrəʊkə/