  Spin-active color centers in solids show good performance for quantum
technologies. Several transition-metal defects in SiC offer compatibility with
telecom and semiconductor industries. However, whether their strong spin-orbit
coupling degrades their spin lifetimes is not clear. We show that a combination
of a crystal-field with axial symmetry and spin-orbit coupling leads to a
suppression of spin-lattice and spin-spin interactions, resulting in remarkably
slow spin relaxation. Our optical measurements on an ensemble of Mo impurities
in SiC show a spin lifetime $T_1$ of $2,4$ s at $2$ K.
