Electron-spin-resonance (ESR) spectroscopy has been applied to vanadium, niobium (and tantalum) complexes with N2 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, S = 1/2, radicals, g values and hyperfine parameters (51V, 93Nb 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.

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