The reactions of bicyclopropylidene 2 with bis(cyclopentadienyl)bis(trimethylphosphane)titanium(II) 1 and with the cobalt chelate 4 give the first known metal complexes of bicyclopropylidene 3 and 7, respectively, in good yields; the bicyclopropylidene cobalt complex 7 is characterized by an X-ray crystal structure analysis which shows a severe out-ofplane bending of the alkenic ligand, indicating considerable back-bonding which goes along with a release of strain in the ligand.

Dedicated to Edgar Heilbronner on the occasion of his 80th birthdayThe electronic structure of spiro [4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron-energy-loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 ± 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron-impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b 1 MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic-absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density-functional and CASSCF/ CASPT2 quantum-chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn-Teller distortion of the anion ground state. Contrary to expectations based on a single-configuration model for the electronic states of 1, it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.1. Introduction. ± Spiroconjugation, a concept first introduced in 1967 [1] [2], results in many interesting spectroscopic and chemical properties. As a consequence, spiroconjugated compounds have often been studied experimentally and theoretically, and they find applications in technology (e.g., as materials for organic light-emitting diodes [3] or nonlinear optical materials).