Carbonyl stretching frequencies and force constants in cyclopentadienylmolybdenum tricarbonyl derivatives

The infrared spectra of compounds of the type RM(CO)3C5H5 (M = Mo or W; R = (CH3)3Sn, σ-alkyl groups, halides, etc.) exhibit the expected three v(CO) frequencies (2A′ + A″). The relative positions of the A″ and the lower A′ frequencies interchange upon variation of the electronegativity of the R group. Values for the Cotton–Kraihanzel stretching force constants k1 and k2 have been calculated from these v(CO) frequencies assuming the ratio kd/ks = 2 for the interaction force constants. From these force constants the σ and π parameters of Graham were calculated. The σ and π parameters were found to be very similar for analogous RM(CO)3C5H5 and RMn(CO)5 compounds.