Biradicaloid compounds

Compound (15) also exhibits a 2-centre, le bond between P atoms and again the bond is long (about 2.76 A), while the unusual biradicaloid (16) is thermally stable up to 150°C. The constraints placed upon the phosphorus atoms in compound (17) result in hypercoordination of the phosphorus adjacent to the oxygen, while (18) constitutes the first cationic phosphorus-carbon cluster, namely nido-[3,5- Bu2-1,2,4-C2P3]. All such observations show great promise for future development. 

Due to its antiaromaticity, cyclobutadiene CBD is not stable under ordinary conditions, but derivatives thereof are, for example from the squaric acid family. Compounds SQ-D and SQ-DMA even possess the possibility for a planar biradicaloid state (within the CBD unit) and a perpendicular one (through twisting of the substituents). The tuning of 3 has also been proposed as a means of generating new long-wavelength absorbing dye systems [98],... 

Figure 6.27. Salem diagram for hydrogen abstraction by a carbonyl compound. The biradicaloid product states are denoted by (dot-

Most surprising is the formation of spiro compounds of type 214 when, contrary to electrostatic laws, formally two positively charged carbon atoms of the methylene groups combine. Quantum-mechanical calculations206,211,212 suggest that this process is favoured by the symmetry of molecule frontier orbitals and it may be asynchronous, occurring via the intermediary formation of a biradicaloid species. 

B and C do not. There is very little reduction in the ring current of D compared to the parent compound A [263,264], whereas B and C show marked reductions in ring current [264,265], Tn the ca.ses of compounds C and D, a detailed consideration of tlic possible Kckul forms suggests that bond localisation should be more marked in C [251] this is in accord with the observed n.m.r. spectra. It has been suggested that in the case of C a biradicaloid form, in which both henzo-rings are delocalised, may also contribute to the overall structure [264],... 

Kubo T, Sakamoto M, Nakasuji K (2005) Biradicaloid character of phenalenyl-based aromatic compounds with a small HOMO-LUMO gap. Polyhedron 24 2522—2527... 

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