Thermolysis of metal carbonyls of the late transition elements has long been known to produce pure metals, a reaction used in metallurgical processing of nickel and the CVD of films of nickel, cobalt, iron, and some alloys. [1,2] Oxides can also be produced from metal carbonyls by conducting the pyrolyses in air. A recent example includes the formation of tetrahedral nanocrystals of CoO from Co 2 (CO) 8 .[3] We report in this paper that substitution of one nitrosyl ligand into the coordination sphere of the cobalt carbonyl completely alters the course of the reaction and the nature of the product. The new solid-state product, CoN x O 1-x , exhibits electrical conductivity many orders of magnitude greater than pure CoO. Due to the thermal sensitivity of the solid-state product, its synthesis is limited to a low-temperature reaction such as described here. Nanocrystalline particles having a coherence length of 14 nm were prepared by passing Co(CO) 3 (NO) (P Co(CO) 3 (NO) = 20 torr) into a vertically oriented hot-wall reactor heated to 350°C. The orange color of the gas mixture at the entrance of the reactor quickly dissipated and was replaced by an aerosol of ultrafine particles that was collected on a medium-porosity glass frit at the end of the reactor. In a typical experiment 1.50 g of Co(CO) 3 (NO) was converted into 0.25 g (40 % yield) of a black powder at a reactor temperature of 300°C over a period of 3-5 min. Deposition on the reactor wall and unreacted precursor (observed in the liquid-nitrogen-cooled trap) account for the moderate yield. If a carrier gas (Ar or N 2 ) was used, its mass flow rate ranged from 0 to 300 sccm, and the residence time within the reactor was of the order of tens of seconds. All references to temperature correspond to the maximum value at the center (in length) along the reactor. When this value was 300°C, the temperatures at the entrance and exits were 70 and 250°C, respectively.Thin films of the oxonitride were deposited using a stainless steel cold wall reactor described in detail elsewhere. [4] Use of the cold wall reactor minimized gas-phase reactions, however, we also found it necessary to reduce the concentration of the Co(CO) 3 (NO) in the gas phase by cooling the precursor vessel. The equilibrium vapor pressure, measured between -13 and 17°C in a closed system with a capacitance manometer, obeyed the equation ln P = 23.3 -(5480/T), where P is in torr and T in kelvin. The precursor was cooled to -23°C during most depositions, and the total pressure was maintained at 3 torr using an argon flow rate of 14 sccm. Smooth films exhibited the same composition and diffraction patterns as found in the nanoparticles isolated from the aerosol process.All reflections in the X-ray powder diffraction pattern ( Fig. 1) for particles prepared between 200 and 400°C were indexed to a cubic lattice with a = 4.415 Å, a value approximately 40 % between that of the metastable zinc blende phases of CoO (4.544 Å) [5,6] and CoN (4.28 Å). [7] The stable rock salt phase of CoO has a lattice constant...