Nitrido-ruthenium cluster [Ru6N] catalysts were prepared by reacting [Ru6N(CO)16](-) cluster with MgO, K(+)-doped MgO or Cs(+)-doped MgO as a potential ruthenium nitride catalyst. The [Ru6N] unit remained in reaction conditions, and exhibited higher activities in ammonia synthesis than conventional Ru catalysts or Ru clusters prepared from [Ru6C(CO)16Me](-) or [Ru6(CO)18](2-).