The electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp-2*Yb are dramatically different from those of the 2,2'-bipyridine adducts. The monomeric phenanthroline adducts are ground state triplets that are based upon trivalent Yb(III), f(13), and (phen(center dot-)) that are only weakly exchange coupled, which is in contrast to the bipyridine adducts whose ground states are multiconfigurational, open-shell singlets in which ytterbium is intermediate valent (J. Am. Chem. Soc 2009, 131, 6480; J. Am. Chem. Soc 2010, 132, 17537). The origin of these different physical properties is traced to the number and symmetry of the LUMO and LUMO+1 of the heterocyclic diimine ligands. The bipy(center dot-) has only one pi(1)* orbital of b(1) symmetry of accessible energy, but phen(center dot-) has two pi* orbitals of b(1) and a(2) symmetry that are energetically accessible. The carbon p(pi)-orbitals have different nodal properties and coefficients and their energies, and therefore their populations change depending on the position and number of methyl substitutions on the ring. A chemical ramification of the change in electronic structure is that Cp-2*Yb(phen) is a dimer when crystallized from toluene solution, but a monomer when sublimed at 180-190 degrees C. When 3,8-Me(2)phenanthroline is used, the adduct Cp-2*Yb(3,8-Me(2)phen) exists in the solution in a dimer-monomer equilibrium in which Delta G is near zero. The adducts with 3-Me, 4-Me, 5-Me, 3,8-Me-2, and 5,6-Me-2-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and L-III-edge XANES spectroscopy as a function of temperature and variable-temperature H-1 NMR spectroscopy.