Two-dimensional (2D) silicanes are promising semiconductors for applications in optoelectronics and photochemistry. Covalent functionalization presents a facile strategy to customize silicanes to attain desired properties. A comprehensive understanding of the impacts of ligands on silicanes is thus pivotal. In this study, we perform density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to investigate the effects of three typical classes of ligands: (1) σ-withdrawing and π-donating, (2) σ-withdrawing and π-withdrawing, and (3) σ-donating and π-withdrawing, on the geometric structure, electronic structure, and band edge excited states of silicanes. Covalent functionalization of silicanes enables a wide range of band edge energies. The band gaps can be tuned between indirect and direct, and the underlying mechanism is explained for the first time. Additionally, TDDFT calculations confirm that the band edge optical absorptions can be adjusted by the ligands broadly from the near-infrared to the visible light region. These desirable properties enhance the functionalities of silicanes. Methodologically, we discuss the applicability of using the conventional one-particle orbital model to describe the excited states of silicanes, and nail down the ∼0.1 eV inaccuracy of the model in describing excited state splittings. We present empirical functions for quick estimations of band edge energies of covalently functionalized silicanes. Through careful comparisons, we justify the use of the carbon-adapted σ/π-donating/accepting indices of typical ligands in silicon chemistry and present a set of silicon-based indices for future prudent applications in inorganic computational studies of silicon-based materials.