The M(CO)(4) fragment can be assigned to be isolobal with both CH3+ and CH2. In order to investigate this ambiguous isolobal assignment, we report calculations on compounds of the type M(CO)(4)L-n, where M is Fe (n = 0), Mn (n = -1), and Co (n = +1) and L is an eta(2) ligand with a pi bond, generally an alkene. The L's are varied in electron-withdrawing ability, and patterns in computed structural parameters are investigated. We report that the equatorial OC-M-CO angle is sensitive to the electron-withdrawing ability of the alkene just as the isolobal prediction suggests. Other structural parameters that vary monotonically with electron-withdrawing ability of the alkene are the "bending back" of the alkene, the metal ligand bond distances, and carbon oxygen bond distances. Changing the metal from neutral Fe to a negatively charged Mn or positively charged Co has the result of increasing and decreasing, respectively, the OC-M-CO angle. Several compounds of Ni(CO)(3)L are also investigated as a further example of the ability of the isolobal concept to yield chemically useful information.