Why Is Methylene a Ground State Triplet while Silylene Is a Ground State Singlet? † Yitzhak Apeloig,* ,‡ Ruben Pauncz, ‡ Miriam Karni, ‡ Robert West, § Wes Steiner, ⊥ and Douglas Chapman* ,⊥ Department of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel, Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, and Department of Chemistry, Southern Oregon University, Ashland, Oregon 97520 Received April 9, 2003 The singlet-triplet energy difference in CH 2 and SiH 2 was calculated at the CASSCF level of theory using large Gaussian basis sets that included f-type functions. The total energy was separated into nuclear repulsion and electronic energy, and the latter was further decomposed into the contributions coming from the two electrons highest in energy (the “frontier” electrons, denoted by “f”) and from all the other electrons (denoted by “c” for “core”). The contribution of the frontier electrons was further decomposed into the following terms: E (f) , which is the sum of the kinetic energy and the attraction energy to the nucleus of the two frontier electrons and their repulsion energy from all other electrons, and E ee (f) , the repulsion energy between the two frontier electrons. The results are used to explain why CH 2 is a ground state triplet (lying ca. 10 kcal/mol lower in energy than the singlet) while SiH 2 is a ground state singlet (with the triplet lying ca. 20 kcal/mol higher in energy). The major conclusions are as follows: (1) In addition to the frontier electrons, the “core” electrons and the nuclear repulsion energy also affect the singlet-triplet gap. (2) About 60% of the singlet-triplet energy difference between CH 2 and SiH 2 of 29 kcal/mol may be attributed to the E ee (f) term. (3) The remaining 40% of the energy difference can be interpreted as resulting from a balance between E (f) , the energy of the “core” electrons (E (c) ), and the nuclear repulsion energy (E nuc ); these terms may be related to the HOMO-LUMO energy difference, which is often used in qualitative discussions of singlet-triplet energy gaps. A detailed discussion of the results is presented. Introduction There has been an upsurge of interest in the last two decades in organosilicon 1 and organogermanium 2 chem- istry, interest which is still growing. Silylenes, R 2 Si, 3 and germylenes, R 2 Ge, 4 are among the most important reactive intermediates in these fields, and their chem- istry has therefore attracted considerable atten- tion. 3,4 In all MH 2 species with six valence electrons the singlet state has two electrons in an orbital of σ-sym- metry (a 1 ). In the triplet state this electron pair is unpaired and one electron resides in an orbital of π-symmetry (b 1 ). A major difference between silylene and germylene on one hand, and methylenesthe iso- electronic lowest congenerson the other, is the multi- plicity of their ground states. Methylene is a ground state triplet with the singlet lying 9.0 kcal/mol higher in energy, 5 while both SiH 2 3,6,7 and GeH 2 4 are ground † This paper is dedicated to Prof. Helmut Schwarz, a stimulating scientist and a wonderful human being, on the occasion of his 60th birthday. * Corresponding author. ‡ Technion-Israel Institute of Technology. § University of Wisconsin. ⊥ Southern Oregon University. (1) For extensive overviews see: (a) The Chemistry of Organic Silicon Compounds; Patai. S., Rappoport, Z., Eds.; Wiley: Chichester, 1989. (b) The Chemistry of Organic Silicon Compounds Vol. 2; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 1998. (c) The Chemistry of Organic Silicon Compounds Vol. 3; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 2001. (2) For general reviews see: (a) Rivie `re, P.; Rivie `re-Baudet, M.; Satge ´ , J. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; 1982; Vol. 2, pp 399. (b) Barrau, J. I.; Escudie ´, J.; Satge, J. Chem. Rev. 1990, 90, 283. (c) Power, P. P. Chem. Rev. 1999, 3463. (d) The Chemistry of Organic Germanium, Tin and Lead Compounds; Patai, S., Ed.; Wiley: Chichester, 1995. (e) The Chemistry of Organic Germanium, Tin and Lead Compounds, Supple- mentary Volume; Rappoport, Z., Ed.; Wiley: Chichester, 2002. (3) For reviews see: (a): Gaspar, P. P.; West, R., Chapter 43, p 2463 in ref 1b. (b) Gaspar, P. P. In Reactive Intermediates; Jones, M., Moss, R. A., Eds.; Wiley: New York, 1985; Vol. 3, Chapter 9. See also previous volumes of this series. (c) Tokitoh, N.; Okazaki, R. Coord. Chem. Rev. 2000, 210, 251. (d) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704. (e) Theory: Apeloig, Y., Chapter 2, pp 167-184, in ref 1a. (f) Karni, M.; Kapp, J.; Schleyer, P. v. R.; Apeloig, Y., Chapter 1, p 1 in ref 1c. (g) Ganzer, I.; Hartmann, M.; Frenking, Chapter 3 in ref 2e. (4) (a) Reference 2a, pp 478-490. (b) Nefedov, O. M.; Kolensnikov, S. P.; Ioffe, A. I. Organomet. Chem. Lib. 1977, 5, 181. (c) Satge ´, J. Chem. Heterocycl. Compd. 1999, 35, 1013. See also refs 3c,f,g. (5) (a) For a general review see: Wentrup, C. Neutral Reactive Molecules; Wiley: New York, 1984. Experiments: (b) McKeller, A. R. W.; Bunker, P. R.; Sears, T. J.; Evenson, K. M.; Saykally, R. J.; Langhoff, S. R. J. Chem. Phys. 1983, 79, 5251. (c) Sears, T. J.; Bunker, P. R. J. Chem. Phys 1983, 79, 5265. (d) Bunker, P. R.; Jensen, P.; Kraemer, W. P.; Beardsworth, R. J. Chem. Phys. 1986, 85, 3724. (d) Jensen, P.; Bunker, P. R. J. Chem. Phys. 1988, 89, 1327. Theory: (e) Yamaguchi, Y.; Sherrill, C. D.; Schaefer, H. F., III. J. Phys. Chem. 1996, 100, 7911, and references therein. (f) Slipchenko, L. V.; Krylov, A. I. J. Chem. Phys. 2002, 117, 4696. (6) Experimental determinations of ∆E ST in SiH2: (a) Kasden, A.; Herbst, E.; Lineberger, W. C. J. Chem. Phys. 1975, 62, 541. (b) Berkowitz, J. L.; Green, J. P.; Cho, H.; Ruscic, R. J. Chem. Phys. 1987, 86, 1235. 3250 Organometallics 2003, 22, 3250-3256 10.1021/om0302591 CCC: $25.00 © 2003 American Chemical Society Publication on Web 07/08/2003