The electronic structures of the first-row transition-metal metallocenes, MCp2 (M = V, Cr, Mn, Fe, Co, and Ni), have been studied using a broad range of density functional methods with flexible double-xi plus polarization (DZP) basis sets. Geometrical parameters of the D-5h and D-5d conformations (and structures of lower symmetry for CrCp2 and CoCp2) were fully optimized. For the ferrocene system, best characterized experimentally, the B3LYP, BLYP, and BP86 methods give structures in good agreement with experiment. For the D-5h-D-5d energy difference, the same three methods predict 0.75 kcal/mol (B3LYP), 0.99 kcal/mol (BLYP), and 1.13 kcal/mol (BP86). The cyclopentadienyl rings are very nearly planar; the angles of the C-H bond out of the Cp ring are less than 1degrees for all metallocenes except ferrocene. The C-H bonds are bent slightly away from the metal for V and Mn, slightly toward the metal for Fe and Ni, and virtually not at all from chromocene. According to the energetic and vibrational analyses, the D5h conformations are found to be the global minima, leaving open the possibility that the D-5d conformations may exist under certain conditions. However, MnCp2 probably exists as a mixture of both D5h and D5d Conformations, because both are genuine minima with only a small energy difference. The predicted B3LYP energy differences (D5h-D5d) for the six metallocenes are 0.29 (V), 0.28 (Cr), 0.13 (Mn), 0.75 (Fe), 0.38 (Co), and 0.23 kcal/mol (Ni). A number of reassignments of experimental vibrational bands are suggested. The molecular orbital energy level diagrams and the electron configurations for the metallocenes are compared. This information, obtained in a consistent manner across the first transition metal series, is helpful for discussion of the bonding characters and the chemical reactivities of these metallocenes.