On the p-Electron Distribution in Biphenylene Analogues by Werner Andres, Harald Günther* 1 ), Maria-Eugenia Günther, Heike Hausmann, Günther Jikeli, Henning von Puttkamer, Hans Schmickler, Jier Niu-Schwarz, and W. H. Eugen Schwarz University of Siegen, Fachbereich Chemie, OCII and TC, D-57068 Siegen Dedicated to Professor Edgar Heilbronner on the occasion of his 80th birthday The bonding situation in a series of biphenylene analogues ± benzo[b]biphenylene and its dication, 4,10- dibromobenzo[ b]biphenylene, naphtho[2,3-b]biphenylene and its dianion, benzo[a]biphenylene, (biphenyl- ene)tricarbonylchromium, benzo[3,4]cyclobuta[1,2-c]thiophene, benzo[3,4]cyclobuta[1,2-c]thiophene 2-oxide, benzo[3,4]cyclobuta[1,2-c]thiophene 2,2-dioxide, 4,10-diazabenzo[b]biphenylene, biphenylene-2,3-dione, benzo[3,4]cyclobuta[1,2-b]anthracene-6,11-dione, and 3,4-dihydro-2H-benzo[3,4]cyclobuta[1,2]cycloheptene ± where one of the two benzo rings of biphenylene is replaced by a different p-system (B) was investigated on the basis of the NMR parameters of these systems. From the vicinal 1 H, 1 H spin-spin coupling constants, the electronic structure of the remaining benzo ring (A) is derived via the Q-value method. It is found that increasing tendency of B to tolerate exocyclic double bonds at the central four-membered ring of these systems favors increased p-electron delocalization in the A ring. The analysis of the chemical shifts supports this conclusion. NICS (nucleus-independent chemical shift) values as well as C,C bond lengths derived from ab initio calculations are in excellent agreement with the experimental data. The charged systems benzo[b]biphenylene dication and naphtho[2,3-b]biphenylene dianion (7 2 ) are also studied by 13 C NMR measurements. The charge distribution found closely resembles the predictions of the simple HMO model and reveals that 7 2 can be regarded as a benzo[3,4]cyclobuta[1,2-b]-substituted anthracene dianion. It is shown that the orientation of the tricarbonylchromium group in complexes of benzenoid aromatics can be derived from the vicinal 1 H, 1 H coupling constants. Introduction. ± Since its successful synthesis [1], biphenylene (1), the first stable derivative of cyclobutadiene, has found considerable interest (for reviews, see [2]). Structural investigations by X-ray [3] and electron diffraction [4], as well as the determination of 1 H, 1 H spin-spin coupling constants [5], soon showed that, despite overall p-electron delocalization, the benzo part of 1 displays a significant alternation of C,C bond lengths ( R(1,8b)  137.2, R(1,2)  142.8, R(2,3)  137.0 pm [4]). From the various resonance structures, those with exocyclic CC bonds at the central four- membered ring are thus dominating [6], a conclusion supported also by the results of recent measurements of the 13 C chemical-shift tensor [7]. Helvetica Chimica Acta ± Vol. 84 (2001) 1737 1 ) E-mail: guenmr@in400.chemie.uni-siegen.de