The Influence of Molecular Host Lattices on Electronic Properties of Orbitally (Near‐) Degenerate Transition Metal Complexes

Transition metal complexes often have low‐lying excited electronic states and, as a consequence, tend to be electronically labile, i.e. , their electronic properties exhibit pronounced sensitivity to external perturbations. Often drastic changes in various spectroscopic properties indicating substantial electronic rearrangements can be induced be relatively weak intermolecular forces as provided by nonpolar solvents or solid molecular host lattices. This behaviour can be explained by crossing of potential surfaces in the vicinity of the absolute minimum. Many physical properties of a given orbitally (near‐) degenerate system depend strongly on the relative magnitude of some characteristic parameters determining the shape of the ground Born ‐ Oppenheimer potential surface(s), e.g. barrier height versus zero‐point energy, distance between minima versus zero‐point amplitude, energy difference between minima, etc. Typical examples are systems exhibiting Jahn ‐ Teller activity, spin‐crossover, mixed valence, exchange coupling and other types of electronic near‐degeneracies. In paramagnetic systems changes in the electronic wavefunction can be most conveniently detected and analyzed by using EPR. spectroscopy. In paramagnetic sandwich complexes we studied two types of orbital degeneracies: Jahn ‐ Teller degeneracies (d 7 ‐systems such as Co (cp) 2 , Ni(cp)   2 +and Fe (cp) (bz), low‐spin d 5 ‐systems such as Mn (cp) 2 ) and low ‐ spin / high ‐ spin equilibria (d 5 ‐systems such as Mn (cp) 2 ). By diluting these complexes and ring‐substituted derivatives in a large variety of diamagnetic host systems we have been able to control the 6 A/ 2 E equilibrium of Mn (cp) 2 by influencing the metal‐to‐ring distance and by changing the degree of ring alkylation; similarly we have been able to vary the relative weights of the two electronic states contributing to the two‐fold degenerate electronic ground state of d 5 ‐ and d 7 ‐systems to a large degree by variation of the local asymmetric fields offered by the lattice sites of the host systems. For comparison the electronic ground state properties of octahedral Cu(II) complexes with CuN 6 CuO 6 chromophores, of V (CO) 6 and tetrahedral VCl 4 were also studied by EPR. between 4K and room temperature in several host systems. Characteristic differences in the details of the temperature and host dependence of the EPR. spectra in all these electronically labile systems can be explained in terms of differences in the vibronic coupling type ( E ⊗ e vs. T ⊗ e, t), the strength of linear and/or quadratic JT ‐coupling and the effects produced by spin‐orbit coupling.