We report a combined experimental and theoretical investigation of nonstoichiometric species, CA1Si(2)(-) and its neutral. Photoelectron spectra of CA1Si2- were measured at two photon energies; three major detachment features were observed with a broad ground-state transition. Ab initio calculations were performed on the CA1Si2- anion, which was found to have two low-lying isomers, a cis-C, and a trans-Ct, species, both with a "Napoleon Hat" type of structure. Electron detachment energies from the anions and spectroscopic properties for both the anions and neutrals were calculated. The cis isomer was found to be the ground state by comparing the theoretical results with the experimental measurements. Evidence of the trans isomer was inferred by comparing the theoretical detachment transitions to the experimental spectra. The neutral ground state, resulting from detaching an electron from the cis isomer, was found to undergo a significant geometry change from the "Napoleon Hat＇＇ structure to an "Eiffel Tower" structure, consistent with the broad photodetachment transition of the ground state, whereas the trans isomer maintained the "Napoleon Hat" structure in both its anion and neutral states. The novel "Napoleon Hat" structure was due to a new three-center bond, which involves the three peripheral ligands, and was a general structural feature of a 16-valence-electron tetraatomic species consisting of elements from the first and second series of the periodic table.