We study the geometry and vibrational spectra obtained by density functional theoretical (DFT) methods for isoelectronic V(CO)(6)(-) and Cr(CO)(6) molecules. We compare many Gaussian basis set options with the B3P86 method for the anion, and we systematically investigate results for both species with the SVWN, B3P86, BP86, and B3LYP methods and fewer basis sets. In general the gradient-corrected methods give good agreement for both geometry and vibrational frequencies, where the method variations are greater than effects of small basis set improvements. We note interesting differences between DFT methods for the isoelectronic neutral and anion species. The metal-carbon distance is sensitive to the method, and this distance increases in the anion by 0.025-0.03 Angstrom for all DFT methods compared with the experimental increase of 0.02 Angstrom. The absolute distance is closest to experiment for the neutral with B3LYP (+0.006 Angstrom) and for the anion with B3P86 (+0.009 Angstrom), where the anion results are too long by 0.009-0.033 Angstrom and the neutral results range from -0.02 to 0.006 Angstrom for the B3P86, BP86 and B3LYP methods. However, the B3P86 and B3LYP methods give good agreement for the C-O distance. The vibrational calculations show about 30 cm(-1) random deviations for the metal-ligand modes while the CO stretching modes show 40-80 cm(-1) deviations. The DFT trends for diatomic CO are maintained for the anion; however, the neutral with B3P86 gives frequencies that change from positive to negative deviations from experiment. The anion has its charge distributed over the whole molecule, but with oxygen and the vanadium having increased electron densities. We compare bonding charge distributions from natural population analysis and the long range interactions from electric moment values and electrostatic potential fitting of charges. The DFT calculations seem to be sufficiently good to allow their use in experimental interpretations involving geometry and vibrational frequencies. The isoelectronic comparison can be useful in testing various aspects of new DFT methods since the anion shows some subtle differences from the neutral molecule.