The geometry and vibrational frequencies of CNN and SiNN in the 3-SIGMA- state have been calculated by local density functional theory. The geometry of CNN has an NN bond (1.202 angstrom) that is shorter than the CN bond (1.252 angstrom). For SiNN, the NN bond is shorter (1.164 angstrom) than that for CNN and the SiN bond (1.749 angstrom) is like an Si-N single bond. The calculated frequencies for CNN (1597, 1273, and 377 cm-1) can account for the lower two observed transitions but can not account for the observed transition at 2847 cm-1 as a fundamental. The infrared intensities are all predicted to be small for CNN. The calculated frequencies for SiNN (1821, 569, and 321 cm-1) can account for the observed spectrum and we note that omega-1 is predicted to have a large infrared intensity. The local density functional results for SiNN and CNN are quite different from previously reported molecular orbital predictions which could not satisfactorily account for the experimental results.