New synthetic procedures have been developed for Me(5)C(5)M(acac), M = Co or Ni. The crystal structures of these 17- or 18-electron monomers show that the compounds are isomorphous and in space group P (1) over bar, with the planes defined by the Me(5)C(5) ring and the M(acac) fragment perpendicular to within 5 degrees. The Me(5)C(5) ring in both molecules has an ene-allyl distortion which is rationalized by the low molecular symmetry (C-s), which removes the degeneracy in the e(+) and e(-) ring orbitals. The electronic structure of the cobalt compound is (2)A, as deduced from magnetic susceptibility and EPR spectroscopy. The electronic structure of the nickel compound depends upon the temperature. In the solid state, the compound is diamagnetic below 150 K, but it becomes paramagnetic with increasing temperature. The H-1 NMR chemical shifts of the compound in solution are nonlinear in temperature, and a plot of delta vs T-1 yields an equilibrium constant of 0.47 at 303 K for the low spin <----> high spin equilibrium. The 20-electron phosphine complexes of nickel can be isolated; Me(5)C(5)Ni(acac)(PMe(3)) is a simple paramagnet with two unpaired spins, but the PEt(3) complex exists in equilibrium with its base-free compound in solution. The cobalt compound does not give an isolable phosphine complex. A simple symmetry orbital model is proposed that accounts for the electronic and molecular structures of these organometallic compounds.