Functionalization of endohedral metallofullerenes has been shown to differ depending on photochemical or thermal pathways. We report that Lu(3)N@I(h)-C(80) reacts with thermally generated bis(2,6-diethylphenyl)silylene with high selectivity and forms monosilylated derivative 1b. Unexpectedly, 1b undergoes photochemical conversion to afford isomer 1a under ambient light. These adducts were characterized using NMR, visible-near-IR spectroscopy, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Single-crystal X-ray structure determination of 1a reveals a rare example of an open 1,2-adduct at the [5,6]-ring junction of the I(h)-C(80) cage. The electrochemical study reveals that the redox potentials of 1a and 1b are shifted cathodically compared to those of pristine Lu(3)N@I(h)-C(80) and that monosilylation is effective to fine-tune the electronic properties of endohedral metallofullerenes as well as empty fullerenes. Density functional theory calculations were also performed, which provide a theoretical basis for the structures and the behavior of the encapsulated Lu(3)N cluster.