All-electron scalar relativistic and nonrelativistic methods at various levels of the quantum chemical theory are employed to investigate spectroscopic constants for the cyanides of group I and group I I metal atoms. The relativistic effect is small for the group I cyanides but much larger for the group I I cyanides and increases as a function of the atomic charge. This can be explained in terms of the relativistic effects on the ionization potentials of metals and the atomic orbital contraction-dilations. The relativistic correction increases the covalent character of the bonding between the metal atom and the cyano radical in general. The spin-orbit effects on the spectroscopic properties are not so large except for the AuCN. (c) 2007 Elsevier B.V. All rights reserved.