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[1] However, apart from tetrahedral structure T, which is observed for high-spin transition metals from the 4th period, [2] sawhorse (SH, C 2v ) and trigonalpyramidal structures (TP, C 3v ) are alternatives (Scheme 1).[3]The highly reactive transient carbonyls [M(CO) 4 ] (M = Fe, Ru, Os) have SH structures, [4a-c] as do a few recently isolated Ru 0 and Rh I complexes, which show remarkable reactivity.Even computationally, the TP structure has been rarely considered. Pidun and Frenking investigated by DFT methods [RhX(CO) 3 ] complexes with X = H, HC = O, CH 2 OH, and OMe. For [Rh(HC = O)(CO) 3 ], TP is the (slightly) preferred ground-state structure, whereas all other complexes have either a P or an SH form in the global minimum.[5] Within the complementary spherical electron density (CSED) model, Mingos identified SH and TP forms as the next best choices to planar structures for 16-electron complexes of late transition metals. [6] In computations using the orbital ranked symmetry analysis method (ORSAM), Bayse and Hall considered d 8 -MH 4 hydrides with SH and TP forms.[7] Proposals for making Scheme 1. Planar (P), tetrahedral (T), sawhorse (SH), and trigonalplanar (TP) structures.

The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C15H11 = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop‒ and trop2−• anions show anti-aromatic properties which are dependent on the counter cation M+ and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C16H12 = dbcot) were prepared, which show classical aromatic character. A d8-Rh(I) complex of trop− was prepared and the structure shows a distortion of the C15H11 ligand into a conjugated 10π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants.

Tetra-tert-butyldisilene and di-tert-butylsilylene (1), generated by photolysis of hexa-tert-butylcyclotrisilane, react with 1,8-di-tert-butylocta-1,3,5,7-tetrayne to furnish a C−C-linked bis(disilacyclobutene) derivative with terminal alkynyl groups 7 and a quatersilirene 8 by addition of 1 to all C−C triple bonds. Upon prolonged irradiation, 8 rearranges partly to the 2,5-disilabicyclo[2.2.0]hexa-1(6),3-diene with terminal silirene groups 9. The structures of 7, 8, and 9 were determined by X-ray crystallography.

[1] However, apart from tetrahedral structure T, which is observed for high-spin transition metals from the 4th period, [2] sawhorse (SH, C 2v ) and trigonalpyramidal structures (TP, C 3v ) are alternatives (Scheme 1).[3]The highly reactive transient carbonyls [M(CO) 4 ] (M = Fe, Ru, Os) have SH structures, [4a-c] as do a few recently isolated Ru 0 and Rh I complexes, which show remarkable reactivity.Even computationally, the TP structure has been rarely considered. Pidun and Frenking investigated by DFT methods [RhX(CO) 3 ] complexes with X = H, HC = O, CH 2 OH, and OMe. For [Rh(HC = O)(CO) 3 ], TP is the (slightly) preferred ground-state structure, whereas all other complexes have either a P or an SH form in the global minimum.[5] Within the complementary spherical electron density (CSED) model, Mingos identified SH and TP forms as the next best choices to planar structures for 16-electron complexes of late transition metals. [6] In computations using the orbital ranked symmetry analysis method (ORSAM), Bayse and Hall considered d 8 -MH 4 hydrides with SH and TP forms.[7] Proposals for making Scheme 1. Planar (P), tetrahedral (T), sawhorse (SH), and trigonalplanar (TP) structures.