Pentalene complexes of ruthenium: two isomeric forms of triruthenium carbonyl clusters with face- and edge-bridging pentalene ligands: crystal structures of two isomers of [Ru3(CO)8{C8H3(SiMe3)3-1,3,5}]

Abstract

The complexes [Ru₃(CO)₈{C₈H₃(SiMe₃)₃-1,3,5}] exist in two isomeric forms which have been characterised by X-ray diffraction. In one of the isomers (A) only two ruthenium atoms of the Ru₃ triangle are directly bonded to the pentalene ligand; the geometry of the Ru₂–pentalene moiety is closely comparable with that established for [Ru₂(GeMe₃)₂(CO)₄(C₈H₆)]. This edge-bridging isomer is fluxional but with a free energy of activation substantially less than that of the unsubstituted species [Ru₃(CO)₈(C₈H₆)]. The two Ru atoms which are bonded to the pentalene each carry two CO groups, while the Ru atom not so bonded carries four. Crystals of (A) are yellow, monoclinic, space group P2₁/c, with Z= 4 in a unit cell of dimensions a= 16.264(8), b= 9.023(3), c= 22.770(9)Å, and β= 97.95(4)°; despite the different activation energy, the angle between the Ru₃ plane and that of the pentalene is much the same as in [Ru₃(CO)₈(C₈H₆)](ca. 50°). In the other isomer (B) all three atoms of the Ru₃ triangle are directly bonded to the pentalene ligand in a symmetrical parallel orientation; this necessitates rearrangement of the CO ligands. The Ru atom which is centrally placed with respect to the pentalene ligand now carries only two CO groups, while the flanking Ru atoms carry three each. Crystals of (B) are orange-red, monoclinic, space group P2₁/n, with Z= 4 in a unit cell of dimensions a= 11.488(3), b= 18.646(7), c= 15.379(7)Å, and β= 90.99(3)°. Both structures have been solved by heavy-atom methods from data collected on a Syntex P2₁ four-circle diffractometer. For (A) the 7 701 unique reflections refined to R 0.039 (R′ 0.047), and for (B) the 3 588 unique reflections gave R 0.037 (R′ 0.044).