Reaction of ground state Na atoms with (CO)-C-13 in an adamantane matrix in a rotating cryostat at 77 K gives a mononuclear monoligand complex whose EPR spectrum consists of an isotropic quartet of doublets with a(Na) = 889 MHz, a(C-13) = 21.8 MHz, and g = 2.0015. This suggests that a Na atom can interact with one CO to generate NaCO with a singly occupied molecular orbital that is made up of Na 3s and C 2s orbitals with ca. 96% unpaired spin population in the Na 3s orbital.