Infrared spectra of unstable species such as CO-free ruthenium tetraphenylporphyrin RuTPP and RuCOTPP (species with vacant coordination sites) isolated in solid argon at 8 K have been recorded. Selective deposition conditions allow the isolation of either RuTPP and RuCOTPP or RuCOTPP and RU(CO)(2)TPP. This depends on the preparation conditions of the sample. A specific Ru-CO bending mode has been characterized at 590.1 cm(-1) for Ru(CO)(2)TPP. The behavior of each vibrational mode of RuTPP, RuCOTPP, and Ru(CO)(2)TPP has been analyzed. Modes such as gamma(8) at 721.3 cm(-1) (out-of-plane stretching mode gamma((C beta-H)sym)) and v(41) at 1342.8 cm(-1) (v(C alpha-N) coupled with delta(C alpha-Cm)) reflect the charge transfer in the porphyrin. Indeed, the addition of one or two CO ligands to RuTPP reduces the charge transfer between the metal center and the porphyrin, which appears as an increase in the frequency of the v(41) mode and in a decrease in that of the gamma(8) mode.