Electronic structure of the half-sandwich complexes [Ln(AlMe4)2CpR] with (Ln=Lu, Y, La), (CpR = 1,3-(Me3Si)2C5H3 and C5Me5), has been investigated using density functional theory method at the ZORA/PW91/TZP level. The study reveals that the twist angle θ decreases with increasing ionic radii and the effect of substitution,1, 3-(Me3Si)2C5H3 on the occupied molecular orbitals, ensuring a great stability. The quantum theory of atoms in molecules (AIM) and energy decomposition analysis indicate that the substitution 1, 3-(Me3Si)2C5H3 increases the degree of covalency in the bonding CpRLn (Lu+3, Y+3, La+3)–(AlMe4)2. According to the Pearson terminology, The La(AlMe4)2(1,3-(Me3Si)2C5H3) complex shows the higher acidity.