The optical and chiroptical properties of Mo(His-N ε-C2H4COOCH3)(allyl)(CO)2 (1) and Mo(His-N ε-C2H4COOCH3)(2-methylallyl)(CO)2 (2) (His = O,N,N δ-l-histidinate, allyl = η-C3H5) and their one-electron-oxidized derivatives 1 + and 2 + are presented. The trends observed in the UV−vis and CD spectra of 1 and 2 in MeOH and MeCN are related to the ratio between the regioisomers a and b, which differ in the position of the allyl ligand relative to the facially coordinating His ligand. Increase of the ratio in favor of a leads to a bathochromic shift of the MLCT transition in the UV−vis spectra. In addition, an increase in the abundance of isomer a leads to a decrease in intensity of the negative Cotton effect around 320 nm and a simultaneous increase in intensity of the negative Cotton effect around 410 nm. CD and UV−vis spectra of 1 + and 2 + in MeCN were acquired via spectroelectrochemical measurements. The electronic transitions observed in the UV−vis spectrum of 2 + at 335 and 546 nm appear to correspond to the negative Cotton effects observed in the CD spectrum at 345 and 553 nm. Similarly, the electronic transition in the UV−vis spectrum of 1 + at 348 nm appears to be related to the negative Cotton effect at 352 nm. To the best of our knowledge, this is the first CD spectroscopic study on paramagnetic organometallic compounds.