Journal of Organometallic Chemistry 646 (2002) 68 – 79 www.elsevier.com/locate/jorganchem Account The quest for triplet ground state silylenes Peter P. Gaspar *, Manchao Xiao, Dong Ho Pae, Daniel J. Berger, Tesfamichael Haile, Tongqian Chen, Deqing Lei, William R. Winchester, Ping Jiang Department of Chemistry, Washington Uniersity, One Brookings Drie, Campus Box 1034, St. Louis, MO 63130 -4899, USA Received 31 August 2001; accepted 22 November 2001 Abstract The scarcity of triplet silylenes compared with triplet carbenes can be understood in terms of the sizes of the valence orbitals. The larger size of the silicon orbitals leads to a decrease in the repulsion of the nonbonding electrons in the singlet state and hence their energy-lowering separation in the triplet state is less capable of compensating an attendant promotion energy. Calculations suggested that the effect of bulky substituents must be supplemented by a reduction of their electronegativity in order to reduce the promotion energy to the point that a triplet ground state can be achieved at an attainable bond angle. The culmination of this approach has been the generation of a silylene (t -Bu) 3 SiSiSi(i Pr) 3 that appears to react from its triplet ground state. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Silylene; Triplet ground state; Reaction mechanisms; Photochemistry; Organosilicon chemistry Mechanistic differences between the lighter elements of organic chemistry and the heavier elements that are the province of main-group chemistry have long inter- ested the Gaspar group. The study of reactive interme- diates like carbenes and their analogs has played an important role in this work. The reactions of these short-lived species are so distinctive that gross differ- ences in mechanisms and reactivity can be deduced from product studies even before formal mechanistic investigations have begun. Here, we examine a dra- matic difference between carbenes and silylenes. Carbe- nes are found with both singlet and triplet ground electronic states, each with a distinctive chemistry [1]. The novelty of such ‘electronic isomers’ that are nearly degenerate while differing in geometric and electronic structure as well as in their reactivity has attracted many chemists to the study of carbenes since the 1950s. But only singlet silylenes are well-documented. A brief review of carbene structures, Fig. 1, is impor- tant background for our story. It has been known for nearly half a century that the lowest singlet and triplet states of carbenes can have nearly equal energies [1]. This is quite understandable for bent carbenes, since separation of the nonbonding electrons requires promotion of an electron from an s-weighted hybrid orbital to a pure 2p atomic orbital. But this one-electron promotion energy is counteracted by a decrease in the electron–electron repulsion of the nonbonding electrons when they are placed in separate orbitals. For the majority of carbenes that are bent, the energy lowering associated with separating the lone- pair electrons can outweigh the one-electron energy increase that attends its movement from an s-weighted orbital to one that is pure p. Clearly, the geometry of a carbene makes a differ- ence. For a linear carbene the nonbonding orbitals would be degenerate and the triplet state lower in Fig. 1. Schematic representations of the lowest energy singlet and triplet electronic states of carbenes. * Corresponding author. Tel.: +1-314-935-6568; fax: +1-314-935- 4481. E-mail address: gaspar@wuchem.wustl.edu (P.P. Gaspar). 0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0022-328X(01)01464-4