The dynamic behavior of [(C6H7)Fe(CO)(3)]BF4 (I) and [(C7H9)Fe(CO)(3)]BF4 (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature H-1 and C-13 NMR spectra indicate that both complexes have a solid state phase transition above which there is rapid reorientation of the cyclodienylium rings and fast exchange of the carbonyl groups. The transition occurs between 253 and 263 K for I and between 329 and 341 K for II. The presence of the phase transition is confirmed by differential scanning calorimetry (DSC). Fe-57 Mossbauer spectroscopy supports the notion that complex I is highly mobile at room temperature, while II is relatively static. The activation energy for the cyclodienylium group rotation in the high-temperature phase of I is estimated from H-1 spin-lattice relaxation time measurements to be 17.5 kJ mol(-1). Static C-13 NMR measurements of the solid complexes in the high-temperature phase indicate that the C-13 chemical shift anisotropies are only 20-30 ppm. This is significantly less than that expected to result from motion of individual groups and thus suggests that rotation of the whole molecule is involved. A single-crystal X-ray structural determination of complex II, at 295 K, showed that the complex is tetragonal (space group P4(1), a = 10.610(1) Angstrom, c = 21.761(3) Angstrom, V = 2449.7(5) Angstrom(3), rho(calc) = 1.734 g cm(-3)), with eight cycloheptadienyl cations and eight tetrafluoroborate anions per unit cell. In addition, powder X-ray diffraction studies of both I and II confirm that at low temperatures both complexes have a tetragonal unit cell, which transforms to a cubic unit cell above the phase transition. The powder patterns, recorded above the phase transition, support the proposal that the complexes are undergoing whole-molecule tumbling in their dynamic regimes.