Nickel atoms have been cocondensed in various 12C16O/14N2, 12C16O/14N2/Ar, and 12C18O/14N2/15/N2/Ar matrices and the infrared and Raman spectra of the products recorded at 4.2–10 °K. In addition to the infrared absorptions associated with the known compounds Ni(N2)4 and Ni(CO)4, other NN and CO stretching modes are observed which can be ascribed to the mixed carbonyl–dinitrogen species Ni(N2)m (CO)4-m where m = 1–3. Confirmation of the molecular constitutions and vibrational assignments of the isolated species was obtained from (a) variable concentration experiments in which either N2 or CO was arranged to be in excess, (b) diffusion controlled warm-up experiments in the range 10–35 °K, and (c) isotope substitution experiments using 14N2/15N2/12C18O/Ar matrices in which the total nitrogen to carbon monoxide ratio was varied. Cotton–Kraihanzel kCO and kNN force constants are computed for the mixed compounds Ni(CO)3(N2), Ni(CO)2(N2)2, and Ni(CO)(N2)3 and trends within the series are discussed in terms of the s-donor and p-acceptor bonding properties of the N2 and CO ligands. An analysis is presented in which the changes in the CO and N2 bond stretching force constants in passing from Ni(CO)m to Ni(CO)m(N2)4-m and from Ni(N2)4-m to Ni(CO)m(N2)4-m are related to the change in s and p orbital populations. The results of that analysis indicate that CO is both a better s donor and a better p acceptor than N2. Integrated absorbances of the bands associated with the species Ni(CO)m(N2)4-m are measured and are used to compute ratios of the transition dipole moments (µco')m and (µN2')m for CO and N2 respectively. The observed trends for (µco')m and (µN2')m as functions of m are found to be in keeping with the changes in the respective CO and N2 p orbital populations.