Cobalt complexes dimeric cyclopentadienyls

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

The structure of the cobalt reagents is worth a mention. Cobalt has nine electrons so the second reagent is easy nine from Co, five from the cyclopentadienyl, and two each from the two COs giving 18 in all. But why is the first reagent a dimer The monomer Co(CO)4 would have 9 + 8 = 17 electrons. complex of Co(0)... 

---