Structures of two derivatives of the curved¯uoradene ring system (C 19 H 12 ) have been determined. Both have phases that are highly pseudosymmetric. At room temperature crystals of 7b-triisopropylsilyl¯uoradene (C 28 H 32 Si) have a P " 1 cell that contains two independent molecules (Z = 2) and that is almost centered. Crystals of 7b-(2,4-dinitrophenyl)¯uoradene (C 25 H 14 N 2 O 4 ) have both a P2 1 /c cell with Z = 1 and a P2 1 /c cell with Z = 2. The molecular volumes in these two P2 1 /c structures differ by 0.7%, but the structures are otherwise virtually the same; the two independent molecules in the larger cell are related by a pseudotranslation. Some of the atomic ellipsoids in the P2 1 /c, Z = 1 structure are very large and eccentric, and there are some hints in the diffraction pattern of an incipient phase transition, but the Z = 1 and Z = 2 phases are clearly different. The P2 1 /c, Z = 2 crystal at 295 K probably contains some volume fraction of the Z = 1 phase; when the temperature is lowered to 273 K this fraction is decreased markedly. The pronounced pseudosymmetry in the P " 1 and P2 1 /c structures that have Z = 2 has been investigated by analysing the atomic coordinates, by performing re®ne-ments in the smaller pseudocells and by making separate Wilson plots for the classes of re¯ections which are systematically strong and systematically weak. All three approaches are informative, but they reveal different information. Leastsquares ®ts of coordinates of corresponding atoms measure the similarity of the molecular conformations. The Wilson plots allow a quantitative comparison of the intensities of the strong and weak re¯ections and thus an assessment of the deviations of the true structure from the smaller pseudocell structure. Comparison of the atomic displacements obtained in the full and pseudocell re®nements shows where the structural distortions are largest and provides an indication of their directions."

The reaction of 1-benzoyl-6-hydroxy-6-phenylfulvene with thallous ethoxide results in the formation of [Tl{1,2-C5H3(COPh)2}]. Subsequent reaction with [Ru(μ3-Cl)(Cp*)]4 gives [Ru{η5-1,2-C5H3(COPh)2}(Cp*)] (3, 79%). Treatment of 3 with hydrazine monohydrate results in the formation of the pyridazine (6, 73%). The structures of 3 and 6 have been determined by X-ray crystallography.

The diethoxycyclopropenylidene complexes [M(CO)5{C3(OCH2CH3)2}] (M = Cr, Mo, W) react with K[Fe(CO)2(Cp)] to give the bimetallic cyclopropenylidene complexes [M(CO)5{μ2-C3(OCH2CH3)}Fe(CO)2(Cp)], but the second ethoxide is not displaced by excess K[Fe(CO)2(Cp)]. Spectroscopic evidence (1H NMR, 13C NMR, and IR) as well as a X-ray crystal structure determination of [Cr(CO)5{μ2-C3(OCH2CH3)}Fe(CO)2(Cp)] suggests that the central C3 ring has a great deal of cyclopropenium character.

Numerous printing errors in the paper by Xia et al. [Acta Cryst. (2001), B57, 507–516] are corrected.