Silylene-Bridged Dinuclear Complexes Having a Triplet Ground State: Photochemical Synthesis and Structural Characterization of Cp2Fe2(.mu.-CO)2(.mu.-SiR)2 (Cp = .eta.5-C5H5; R = 2,4,6-C6H2iPr3, 2,6-C6H3Et2, and Mesityl)