The new [Co(btmgn)Cl₂] complex and the 1,8-bis(tetramethylguanidino)naphthalene (btmgn) ligand were synthesized and characterized. The X-ray single crystal investigation showed distorted tetrahedral geometry around the Co(II) ion. The geometry of the btmgn and [Co(btmgn)Cl₂] complex was optimized using the B3LYP/6–311G(d,p) method. The calculated geometric parameters at the optimized structure of the [Co(btmgn)Cl₂] complex showed good agreement with our reported X-ray structure. The two tetramethylguanidino groups are in a cis-type position to the naphthalene ring plane both in the free and coordinated btmgn. The large red shift of the 𝜈_C=N mode upon coordination indicates the strong ligand–metal interactions. The calculated natural charges using natural bond orbital (NBO) analysis at the two coordinated Cl-atoms are not equivalent. Also the two LP(4)Cl$\rightarrow$LP*(3)Co intramolecular charge transfer interaction energies (E⁽²⁾) are 29.00 and 39.17 kcal/mol, respectively. The two Co-Cl bonds are not equivalent where the longer Co-Cl bond has more electronegative chlorine atom than the shorter one. Molecular electrostatic potential (MEP) study of the btmgn ligand showed that the N4 and N7 atoms are the most reactive nucleophilic centers for the coordination with the Co²⁺ ion. The [Co(btmgn)Cl₂] complex has higher polarizability (𝛼₀), first hyperpolarizability (𝛽₀) and lower energy gap (𝛥E) than the free ligand. The TD-DFT calculations predicted the transition bands at 337.2 nm (f=0.2299, H$\rightarrow$L) and 342.6 nm (f=0.1465, H-2/H$\rightarrow$L) for the btmgn and [Co(btmgn)Cl₂], respectively.