Synthesis and Oligomerization of CpM(CO) 2 .

We showcase efficient synthetic protocols of cyclopentadienyl metal dicarbonyl, CpM(CO) 2 (M = Rh and Ir). Reflecting the relativistic effect, the 1 H and 13 C signals of the Cp ring in CpIr(CO) 2 were upfield shifted when compared with the Rh analogue. A missing dinuclear complex, (CpIr) 2 (μ-CO)(CO) 2 , was spontaneously generated together with [CpIr(CO)] 3 by the loss of CO. The crystallographic analyses unambiguously determined their unique structures with one and three Ir-Ir bonds, respectively.