Methyleneboranes are stable compounds when their reactive C=B bonds are shielded by sterically demanding substituents and the electron deficiency at the dicoordinate boron atom is reduced through delocalization of adjacent electron pairs. In the classical amino-(methy1ene)boranes of Noth and Paetzold, the electron deficiency at the dicoordinate boron atom is relieved through K-z delocalization of a neighboring nitrogen atom's formally nonbonding electron pair, that is, through formation of a classical two-center, two-electron (2c-2e) bond. In nonclassical methyleneboranes, which are the focus of this review, the electrondeficient center at the dicoordinate boron atom forms nonclassical three-center, two-electron (3 c-2e) bonds with neighboring 0 bonds. These methyleneboranes exhibit structural and spectroscopic properties similar to carbenium ions having strong O-K delocalization (hyperconjugation, bridging) and to transition metal complexes having strong agostic interactions.In classical methyleneboranes, as in the amino-substituted vinyl cations (ketiminium ions) 4,18] the electron deficiency at the dicoordinate atom is greatly reduced through 71-K delocalization of the formally nonbonding electron pairs on the neighboring atom (Scheme 1). Typical examples are the 4 \ 9/ \ @ J /c=B-c \ -/c=B=c \ 6 7 Scheme 1. x-rc Delocalization in classical methyleneboranes and related compounds. Telefax: Int. code + (6421)28-5547 0 8 Y = C R2 9 Y=ML, Scheme 2. Nonclassical 3c-2e bonds through interaction ofelectron-deficient centers with neighboring (5 electrons: the a-agostic type.These interactions are called c ( agostic effectst7] if the electron-deficient center is a metal, but if it is a positively charged carbon atom, the name C-X hyperconjugation is used. Pronounced effects in carbocations are termed nonclassical bridging. The expression O-7c delocalization1' is employed as the generic term. Depending upon the extent of the shift of the bonding electrons in the C-X bond towards the electron-deficient center-the X nucleus must follow for electrostatic reasons-a continuum'' 31 in the degree of 3 c-2e bond development can be observed. The following series of formulas show examples of carbocations having no appre-Angrbr. Chem. In! Ed. Engl. 1993. 32, 985-1009 ((? VCH Verlugs~e.~.seIlschuf~ mhH, 0-69451 Weinheim, 1993 0570-0833193/0707-0985 $10.00+ .2510 14* 1 4 15' 15 o e/' n-n .. /x \ Scheme 4. Nonclassical 3c-2e bonds from interaction of electron-deficient centers with neighboring o electrons: the P-agostic type.--C=B f) -C-B \ 2 /c=B-11 1 c,d 20a.b a,c,e Aryl= Dur b,df A w l = Mes Our I l e , f Me,Si-C~C-SrMe, Dur = duryl = 2.3.5.6-tetramethylphenyl; M e s = mesityl = 2,4,6-trtmethylphenyI The [2 + 21 cyclo adduct 22,[3b1 obtained from the reaction of the amino(imin0)borane Me,Si(tBu)N-B=NtBu with 988

Delocalization in doubly aromatic compounds: Bishomotriboriranide 1 is the first carbene analogue of boron (a borenate ion) characterized in solution (NMR) and in the solid state (X‐ray). With the 3c–2e π bond and 3c–2e π bond that involves the borenate center of 1* and the neighboring BB unit, 1 belongs to the class of doubly aromatic compounds. The (hypothetical) classical borenate 1* without these delocalizations is estimated to be about 90 kcal mol−1 higher in energy than 1 (Dur = 2,3,5,6‐tetramethylphenyl).

phane (6)15], or alternatively undergoes self-condensation to the cyclotetraphosphane (7)16] (steps e orl; respectively). The dissociation of (2) to its thermodynamically stable endproducts (6) and (7) shows a characteristic solvent dependence; a polar medium accelerates formation of the cyclophosphane, and simultaneously causes a shift of the "P{'HJ signal of (2) to low-field.The diphosphane (5) could not be isolated as a reactive intermediate until now. Its identification by 3'P-NMR spectroscopy in solution (significant AB ~pectrum''~) was corroborated by independent synthesis (step g) from (4) and hexachloroethane using an analogous procedure to that of step a.Silyl-and halogen-functionalized phosphanes are, in addition, suitable educts for gas phase pyrolysis, a process which may hinder the intermolecular condensation discussed here. In this way, (2) could form a phosphanediylfS1 via a 1,l-elimination and (5) an analogue of an azo compound via a 1 ,2-eliminationfio1. Procedure A solution of c2cl6 (22.0 g, 93 mmol) in toluene is slowly added to a stirred solution of (1,~'~~ (21.8 g , 93 mmol) in 30 mL toluene under ice cooling. The mixture is stirred for 30 min at room temperature. The toluene solution contains (2) in practically quantitative yield. After purification by distillation, (2) 3.7 g (20% yield), b.p. 28 "C/O.O5 torr) remains in the distillation receiver (-78 "C) (half-life of (2) at 34°C: 13 rnin from 31P{'H}-NMR spectrum).

tri-cis 3 ZEZZ X= Me, Et, Ph all-cis 4 zzzz di-cis 5 EZZE tri-cis 3' ZZEZ allcis' 4' di-cis' 5'between the degenerate conformers di-cis(EZZE) 5 and dicis'-(EZZE) 5', which is frozen at -96°C.The ' H-NMR spectroscopic findings were corroborated by X-ray structure analyses of l b , Id, and 2d. In the crys-The structure of 3 follows from 'H-, I3C-and "B-NMR spectral2'. Signals for the two C-atoms of the C2B2 skeleton appear below -40°C at 6 = 115.2 and 25.0 in the region for sp2 and sp3 C-atoms; both signals show line-broadening characteristic of a C-atom bound to boron. Hence the structure of a 2,3-diboramethylenecyclopropane'3' for 2 can be ruled out. Below -11 "C the B atoms give rise to [**I This work was supported by the Fonds der Chemischen Industrie. We thank Dr. P. H. M. Budzelaar and Dr. S. M. van der Kerk. Utrecht, for the first hints on the structure of the compound 3, and Prof. K. Dehnicke, Marburg, and Prof. P. u. R . Schleyer, Erlangen, for valuable discussions.

The stable stannanediyl 2 and the carbene‐like species 1 react to give stannaefhene 3. The structure of 3 was established by NMR spectroscopy and by X‐ray structure analysis. The very high‐frequency 119Sn‐NMR signal indicates that the C=Sn bond has strongly polar character (ylide form 3A). R1 = Si(CH3)3; R2 = C(CH3)3; R3 = CHR 21.