In order to derive information on chemical bonding, spin equilibria, and the dynamic Jahn-Teller effect in orbitally degenerate sandwich molecules, the d 5( 2E 2g) metallocenes Mn(cp) 2 and Fe(cp) 2 + as well as the d 7( 2E 1g) metallocenes Co(cp) 2 and Ni(cp) 2 + have been diluted in a large variety of diamagnetic host systems and have been studied by EPR between 4 and 300 K. Ring-substituted derivatives and related metallocarboranes as well as some orbitally nondegenerate metallocenes were included for comparison. Both substituted and unsubstituted Mn(cp) 2 can be found in high-spin ( 6 A 1 g ) as well as in low-spin ( 2 E 2 g ) ground states, depending on the molecular environment. Consistent with ligand field theoretical expectations the high-spin ground state is found preferentially in host systems favoring lare metal-to-ring distances (e.g., Mg(cp) 2), while the low-spin ground state is induced by sandwich matrices exhibiting shorter metal-to-ring distances (e.g., Fe(cp) 2, Ru(cp) 2, Os(cp) 2). Both the zero-field splittings of the high-spin species and the g tensors of the low-spin species showed strong dependence on the host lattice and on the temperature. The pronounced host lattice dependence of the g tensors observed for all orbitally degenerate d 5 and d 7 sandwiches contrasts striking with the insensitivity of the magnetic parameters of orbitally nondegenerate systems, e.g., vanadocene ( d 3, 4 A 1 g), to variations in the molecular environments, and can be explained almost entirely by changes in the low-symmetry components of the matric potentials splitting the orbital degeneracy of the guest molecule by different amounts (δ). The determination of the vibronic orbital angular momentum quenching factor V as a function of the environmental splitting parameter δ showed that the dynamic Jahn-Teller motion of the free gaseous guest molecules is gradually suppressed by increasing asymmetric matrix fields. By systematic variation of the host lattices and of the ring substituents we were able to observe a stepwise transition from a purely dynamic Jahn-Teller effect to the static distortion limit in these sandwich complexes. Extrapolation to the free molecule case (δ = 0) shows that for all orbitally degenerate d 5 and d 7 systems studied the Jahn-Teller distortions in the C 5H 5 rings remain entirely dynamic; i.e., the Jahn-Teller stabilization energy E JT never exceeds the energy of a quantum hv of the corresponding active vibrational modes substantially. The degree of covalent delocalization of the singly occupied degenerate metal 3 d orbital over the ligand rings correlates well with the observed Jahn-Teller distortion increasing strongly along the series Fe(cp) 2 + < Mn(cp) 2 < Co(cp) 2 < Ni(cp) 2 +. Comparison of these EPR results with the predictions of semi-empirical MO calculations and with M-C and C-C vibrational amplitudes from electron diffraction indicates that the symmetric e 1 g ring tilt mode is mainly Jahn-Teller active in d 5 metallocenes, while in the d 7 case the e 2 g C-C strecth and the e 2 g ring distortion mode give comparable contributions.