FTIR spectra of tricarbonyl(eta(6)-benzo-15-crown-5)chromium(0) (1) in the presence of lithium, sodium and potassium perchlorate salts in methanol show different responses in the Cr-CO vibrational region of the spectrum. Data from the symmetric (nu(sym)) and antisymmetric (nu(asym)) Cr-CO vibrational stretching modes have been analysed by principal component analysis (PCA) to generate a factor score plot that provides a visual representation of these differential responses. X-ray crystallographic data for the sodium perchlorate complex 1 center dot Na+ and dimensions from DFT-derived structures of 1 center dot Li+, 1 center dot Na+ and 1 center dot K+ indicate that binding M+ in the crown causes electron density and structural changes in the [O(4)-C(9)-C(4)-O(8)]Cr-C(1)=O(1) sections of 1, which vary depending on the nature of the cation. This suggests a mode of action in which Li+ associates with a more compact O(4)C( 9)-C(4)-O(8), while Na+ and K+ differ crucially in the extent of sigma and pi contributions to their effect on nu(sym) and nu(asym). A comparison of the FTIR data from 1, tricarbonyl(eta(6)-1-phenyl-1-aza-15-crown-5)chromium(0) (2) and tricarbonyl(eta(6)-2-phenyl-15-crown-5)chromium(0) (3) with a wider range of cations (NH4+, Li+, Na+, K+, Rb+, Cs+, Mg2+, Ba2+) and anions (AcO-, BPh4-, Br-, C1O(4)(-), I-, SCN-), showed that 1 and 3 both responded significantly to the different metal cations, but 2 did not. The relative cation differentiation of 1, 2 and 3 was measured using the parameter Delta(R(cation)), and ratios of Delta(N(cation)) values [calculated from Delta(R(cation))] distinguished different effects in the FTIR spectra of 1 and 3 for different pairs of cations.