But the active catalyst is most likely the 16e- HCo(CO)3 complex. The low probability of direct alkene reaction with the 18e- saturated HCo(CO)4 catalyst is consistent with the reduced activity at higher CO partial pressures. One can also explain the lower regioselectivity at lower CO pressure by proposing that alkene isomerization is more facile with the resulting 16e- RCo(CO)3 species that results after reaction with alkene as shown below: