Electron-spin-resonance (ESR) spectroscopy has been applied to vanadium, niobium (and tantalum) complexes with N-2 and CO in the corresponding pure solid matrixes at 4 K. Except for V(CO)(6), none has previously been studied by ESR. For these d,(5) S = 1/2, radicals, g values and hyperfine parameters (V-15, Nb-93 nuclei) are obtained and interpreted in terms of axial molecules, distorted from octahedral symmetry by Jahn-Teller effects. There are significant differences with previous work, and the new metal hyperfine data in the two types of complexes are informative.