As a general trend, the tri-atomic isomers prefer doublet spin states where as ground states of monomer and linear dimers are triplets. Both the nitrogen-excess and metal-excess triatomic clusters show minimum energy configurations to be approximately linear. The most stable isomer of Al₂N₂ and Ga₂N₂ is a rhombus with a singlet spin state. The strong dominance of the N-N bond over metal-nitrogen and metal-metal bonds is found to control the structural skeletons and the chemistry of these clusters. The vibrational modes and frequencies are also explained in terms of diferent bond strengths found in the diatomic clusters.