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In order to determine quantitative information about chemical bonding in metallocenes, we investigated the electronic structure of the cobaltocene molecule by EPR and magnetic susceptibility measurements at 4.2°K and by optical spectroscopy at 77°K. Diamagnetic ruthenocene, weakly paramagnetic nickelocene, and paramagnetic cobaltocene single crystals as well as polycrystalline ferrocene served as host systems. From the poorly resolved optical spectra of pure cobaltocene, approximate ligand field parameters were determined. The magnetic properties (g tensor, cobalt hfs tensor) of the lowest Kramers doublet are explained in terms of the relative magnitudes of (a) spin-orbit coupling, (b) static orthorhombic distortion, and (c) vibronic coupling (dynamic Jahn-Teller effect) in the orbitally degenerate 2E1g ground state. From the analysis of the EPR data of cobaltocene-doped ruthenocene, we conclude that covalency effects and vibronic interactions (``Ham effect'') are of comparable importance resulting in a drastic modification of the magnetic parameters compared to a free Co2+ ion in a static crystal field. In agreement with earlier qualitative and semiquantitative predictions, considerable covalency of the singly occupied e1g* orbital (42%± 5% ligand, 58%± 5% cobalt 3d character) was found. The strong change of the EPR parameters going from the ruthenocene to the ferrocene host lattice originates mainly in a strongly enhanced static orthorhombic splitting parameter in the tighter packed ferrocene environment. In cobaltocene single crystal, magnetic dipole-dipole interactions broaden the EPR lines beyond detection even at 2°K. Nickelocene, an S =1 case with a large positive zero field splitting, behaves as a pseudodiamagnet at liquid helium temperature; exchange interactions with the cobaltocene dopant cause significant modifications of the g values but leaves the cobalt hfs tensor almost unaffected.

Aluminum and gallium atoms have been trapped in Ne, Ar, Kr, and Xe matrices and studied by optical and ESR spectroscopy at 4.2 °K and slightly higher temperatures. The results indicate that both metal atoms occupy axially distorted substitutional sites in all rare gas lattices. This elongated tetradecahedral MeX12 coordination is particularly stable for rare gas complexes of Group III metal atoms exhibiting a single unpaired electron in their outermost p shell. From the ESR data large splittings of the aluminum and gallium p shells have been derived increasing from [inverted lazy s] 1600 cm−1 in neon to [inverted lazy s] 3200 cm−1 in xenon for both atoms. The corresponding Jahn-Teller stabilization energies EJT (increasing from [inverted lazy s] 1.5 kcal/mole for MeNe12 to [inverted lazy s] 3.0 kcal for MeXe12) can be explained by the ``σ-π'' effect: The van der Waals interatomic correlation energy is maximized, and the repulsive exchange energy is minimized by attraction of the equatorial ligand atoms to the metal center and repulsion of the remaining ligands from the σ antibonding axial positions. The 2S ← 2P[(n + 1)s ← np] electronic transitions are shifted by [inverted lazy s] + 1000 cm−1 (MXe12) to [inverted lazy s] + 6000 cm−1 (MNe12) relative to the free metal atom values. The ESR spectra exhibit axial symmetry, show effects of preferential orientation, and demonstrate almost complete quenching of the free atom angular momentum in each case. The basic features of the g values and the metal hyperfine tensor (and of their strong dependence on the matrix and on temperature) can be understood within a simple crystal field model, but there are significant deviations. The introduction of orbital angular momentum and spin-orbit reduction factors resulting from orthogonalization of the metal p orbitals to the valence shells of the surrounding rare gas atoms removed a large part of the discrepancies, but quantitative agreement with experiment could be obtained only when the dynamic Jahn-Teller effect was taken into account. In order to establish the geometries of the rare gas cages surrounding the trapped metal atoms, numerical calculations of orbital and spin-orbit reduction factors were performed for various sites in the rare gas lattices. For the determination of the vibronic quenching parameters a slight extension of Ham's second order theory of an orbital triplet interacting with an e2g vibrational mode was required. Our results indicate a remarkable stability of the Al and Ga rare gas complexes. Indeed, from the results of Baylis' semiempirical calculations it can be concluded that atoms with singly occupied p shells form the strongest van der Waals complexes with rare gas atoms among all atoms in the periodic table.

SummaryTransition 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 by 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 JahnTeller 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 (d7-systems such as Co (CP)~, Ni (cp)? and Fe (cp) (bz), low-spin d5-systems such as Mn (CP)~) and low-spin/high-spin equilibria (d5-systems such as Mn (cp)Z). By diluting these complexes and ring-substituted derivatives in a large variety of diamagnetic host systems we have been able to control the 6A/2E equilibrium of Mn ( C P )~ 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 d5-and d7-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 (11) 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 0 e vs. T 6) e, t), the strength of linear and/or quadratic JT-coupling and the effects produced by spin-orbit coupling.

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