The characterization data for 1 and 2 are consistent with their formulations . [ 17 ] The vivid orange color of 2 ( both in the solid - state and in solution ) is noteworthy for a 6d 0 5f 0 metal complex . The electronic absorption spectra of 2 exhibits a broad , intense electronic absorption band from the UV to visible wavelength range , and a strong absorption between 450 and 550 nm . Since 1 is pale yellow and the 6d 0 5f 0 electronic configuration of Th IV precludes metal - localized ff , d - f , and d - d transitions , and on the basis of definitive prior work , [ 12c ] we conclude this transition is due to a spin - allowed but orbital - forbidden p ? ( N)!p * ( N = C ) and ligand - to - metal charge transfer ( LMCT ) .