Single-excitation configuration interaction (CTS) and time-dependent density functional (TD-DFT) calculations have been carried out on the excited singlet states of tetracoordinate trisilane [(CH)(3)Si](2)Si(Cl)CH3 (2) and pentacoordinate trisilane [(CH3)(3)Si](2)Si(Cl)CH2N(CH3)(C=O)CH3 (5) to explain the remarkable red shift of the lambda(max) (Delta lambda(max) = 10 nm) exhibited by 5. The calculated lambda(max) of (H3Si)(2)Si(Cl)CH2NHCHO (3) was red-shifted by 11.2 nm with respect to that of (H3Si)(2)Si(Cl)CH3 (1), in agreement with experiment. In both compounds, the strongest absorption originates from the sigma(SiSi) --> sigma*(SiSi) transition. Compared with that of 1, the sigma(SiSi) orbital of 3 was substantially destabilized, whereas the sigma*(SiSi) orbital was slightly changed. With the additional calculations on tetracoordinate model compounds having an Si-O bond, we concluded that the red shift of the UV spectrum of 5 is caused by the antibonding interaction of the p(pi), orbital on the oxygen atom and the sigma(SiSi) orbital of the same symmetry.