Metal–carbonyl and metal–nitrosyl complexes. Part IX. Molecular structures of cis- and trans-di-µ-dimethylamido-bis(cyclopentadienyl-nitrosylchromium)

Abstract

The molecular geometries of cis- and trans-di-µ-dimethylamido-bis(cyclopentadienylnitrosylchromium), [(π-C₅H₅)Cr(NO)NMe₂]₂, have been determined by X-ray crystal-structure analysis, and compared with the results previously obtained for trans-[(π-C₅H₅)Cr(NO)SPh]₂. The trans-[(π-C₅H₅)Cr(NO)NMe₂]₂ molecule is situated around a crystallographic centre of symmetry, and its (CrN)₂ ring is therefore planar. In the cis-molecule the fold of the (CrN)₂ ring about the N–N vector is 177·8 ± 0·8°, and a departure from planarity is not significantly established. The Cr–NMe₂ bridging bonds are 2·01 ± 0·01 Å(cis-isomer) and 1·99 ± 0·01 Å(trans-isomer). The Cr–Cr distance is 2·72 ± 0·01 Å in the cis-isomer and 2·67 ± 0·01 Å in the trans-isomer, whereas the metal–metal distance in trans-[(π-C₅H₅)Cr(NO)SPh]₂ is 2·95 Å. The N–Cr–N′ angles in the (CrN)₂ rings have an average value of 95·3°, notably smaller than the S–Cr–S′ angle of 100·9° in trans-[(π-C₅H₅)Cr(NO)SPh]₂. The Cr–NO bonds (1·63 ± 0·02 Å) have appreciable double-bond character, and the Cr–N–O valency angles are close to 170°. In cis-[(π-C₅H₅)Cr(NO)NMe₂]₂ one of the cyclopentadienyl rings is randomly distributed over at least two distinct orientations in the same plane. The Cr–C(cyclopentadienyl) bond lengths to the ordered C₅H₅ rings in the cis- and trans-molecules range from 2·21–2·32 Å, mean 2·26 Å. Crystals of trans-[(π-C₅H₅Cr(NO)-NMe₂]₂ are orthorhombic, space group Pccn, with Z= 4 in a cell of dimensions a= 14·94, b= 11·53, c= 9·69 Å. Crystals of cis-[(π-C₅H₅)Cr(NO)NMe₂]₂ are monoclinic, space group P2₁/c, with Z= 4 in a cell of dimensions a= 8·52, b= 14·73, c= 14·72 Å, and β= 114°51′. The atomic parameters were obtained by Fourier and least-squares methods, and the final values of R are 0·105 over 1496 reflections for the cis-, and 0·114 over 662 reflections for the trans-complex.