The 18-electron (18-e(-)) rule is typicallyrestrictedto predicting the stability of transition-metal-based complexes. Herein,we report the use of the 18-e(-) to predict the stabilityin a family of intermetallics MNiSn (M = V, Cr, Fe, and Co) crystallizingin the Co1.75Ge structure type. Site deficiencies at theM site obtained from single-crystal X-ray diffraction are understoodas attaining a stable noble gas electronic configuration. The densityfunctional theory -based structure calculation confirms that the deficientstructure is more stable than the ideal occupation available at thecrystal lattice. MnNiSn, which crystallizes in the half-Heusler crystalstructure, depicts the role of covalent radii of the constituent elementsin determining the crystal structure. Using X-ray absorption spectroscopyand X-ray photoelectron spectroscopy, the local structure of the above-mentionedcompounds was also elucidated, supporting the role of deficiency tunedvalence fluctuation to attain a 18-e(-) configurationthat eventually leads to the formation of stable compounds.

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