As a new perspective for the stability of the layered intercalation compounds, we focused on the hard and soft acids and bases (HSAB) principle, which is primarily used in complex chemistry to explain that hard acids tend to form ionic bonds with hard bases, while soft acids tend to form covalent bonds with soft bases due to the spread of electron clouds. The HSAB principle has been widely employed in complex chemistry to understand the stable ligand-central atom combinations, and various quantitative indices have been proposed, such as the chemical softness (η) introduced by Parr and Pearson. There are few reports where the HSAB principle has been used for understanding the crystalline systems. For example, in binary ionic crystalline materials, the value of η calculated from two elements and the sum of their ionic radius have been utilized to quantitatively evaluate lattice energy. However, this idea is applicable only to the binary materials, and most researches of the stability quantification using the HSAB principle are limited to the complexes. Nevertheless, considering the intercalant as the complex center and the host as the ligand, the relationship between the two is similar, and the importance of a covalent electron density distribution around the intercalant has been emphasized thorough the systematic simulation of GICs. Thus, we believe that the concept of the HSAB principle could be applicable for understanding the stability of layered intercalation compounds as well.