Sarah A. Jackson

Three- and four-co-ordinate copper(I) complexes: 1:1 and 1:2 1-cyanoguanidine–copper(I) halide adducts

The air- and moisture-stable copper(I) complexes Cu2X2·cnge and CuX·cnge·H2O (X = Cl, Br or I) have been prepared by direct reaction of CuX and 1-cyanoguanidine (cnge) in organic solvents and by reduction of aqueous solutions of CuX2(X = Cl or Br) containing cnge with sodium sulfite. Single crystals of Cu2Cl2·cnge 1, Cu2Br2·cnge 2 and CuBr·cnge·H2O 3 were obtained from the latter route. Complexes 1 and 2 are isostructural and comprise mutually perpendicular chains of [XCu(cnge)]n and (CuX2–)n joined at a common halogen. The [XCu(cnge)]n chain, which lies in the mirror plane of the Pbcm space group, is novel. The cyanoguanidine molecule uses both its nitrile and imino nitrogen atoms to bridge the copper atoms which complete their co-ordinatively unsaturated, trigonal-planar, geometry with the common halogen. The (CuX2–)n chain is unremarkable. Complex 3 is characterised by a zigzag (CuBr)n chain and a buckled [BrCu(cnge)]n chain joined at a common copper atom; an unco-ordinated water molecule completes its structure. The [BrCu(cnge)]n chain is similar to that in 1 and 2. The mam difference is the buckling which results in a dihedral angle between adjacent cnge molecules of 8.9°. The (CuBr)n chain consists of bridging bromine atoms strongly bonded to one copper and weakly bonded to the other, giving a trigonal-pyramidal copper(I) co-ordination sphere. Extended-Huckei calculations have been used to examine the factors controlling the choice of a trigonal-planar over a tetrahedral structure.

The origin of structural variety of alkyne complexes of d8 metals. An example of structural isomerism

Abstract The X-ray structures of three metal alkyne complexes are presented: Ir(PMe2 Ph)3(MeC2Me)+, M(CO)4(F3CC2CF3) (M = Ru, Os). The iridium(I) complex adopts an unusual structure (“CS”) which differs notably from that expected for four-coordinate d8 ML4 square-planar complexes. The structure resembles that of a square pyramid, with essentially Cs symmetry, with a short apical MP bond (2.236 vs 2.31 A for the basal MP bonds), short MC (2.01 A) and long CC bonds (1.306 A), which indicate a strong metal—alkyne interaction. The ruthenium(O) and osmium(O) complexes adopt the expected pseudo-octahedral structure. The unusual CS structures, previously reported for cobalt(I) and iron(O) complexes, contrast with the square-planar structure of platinum(II) alkyne complexes. Fluxionality is present in the iridium complex, as well as in those of cobalt, and is attributed to a rapid intramolecular site exchange among inequivalent phosphorus ligands. The iridium complex reacts with additional ligands like alkynes and H2. Extended Huckel calculations are presented to rationalize the structural and reactivity aspects of the whole family of d8 ML3(alkyne) species (M = PtII, IrI, CoI, FeO). It is shown that the four-electron donation of an alkyne ligand is responsible for the unusual CS structure of certain of these ML3(alkyne) species. The alkyne moieties induce a four-electron destabilization in the square-planar structure which is large for diffuse d orbitals such as those of iridium(I), cobalt(I), iron(O), but not of platinum(II). The fluxionality of the complexes is shown to be associated with an easy rotation of the alkyne about the metal—alkyne midpoint, accompanied by a breathing motion of the ML bonds of the ML3 metal fragment. It is shown that unsymmetrical alkynes stabilize a conformation of the complex in which the ligand is turned by 90° with respect to the one presented here. The reactivity towards additional ligands is shown to be associated with the presence of a low-lying LUMO pointing away from the apical bond. Effects of substituents at the alkyne and of ligands at the metal are discussed. It is shown that it is residual four-electron destabilization in M(CO)4(alkyne) complexes which is responsible for the lability of CO ligands and thus for the ligand exchange process.

Infrared spectroscopy and chemistry in liquid rare-gas solvents

This meeting is about ‘unstable’ species studied at low temperature, mostly via matrix isolation or nozzle beams. This article describes a different approach, the use of liquid rare gases (LRG) as low-temperature solvents for investigating unstable species, largely via Infrared spectroscopy, particularly those species involved in organometallic reactions. As an intorduction to the technique two examples of its application to stable compounds. are described; the i.r. spectra of SF6 in liquid Ar, of relevance to work in beams and matrices, and the thermodynamics of the gauche–trans isomerisation of CH2ClCH2Cl. The main part of the paper concerns the generation of novel η2-H2 complexes of transition metals, which is achieved by photolysis of parent metal carbonyls in H2-doped LRG. In particular we are concerned with the bonding of the η2-H2 moiety to the transition metal, and the nature of this non-classical interaction can be probed by isotopic and temperature effects on the ν(H—H) i.r. band.

Photochemical generation and reactions of rhodium (η5-cyclopentadienyl) (ethene) (dinitrogen) in liquid xenon

Abstract Photolysis of CpRh(C 2 H 4 ) 2 dissolved in N 2 -doped liquid xenon at 173 K causes dissociation of ethene and production of CpRh(C 2 H 4 )N 2 (FTIR detection): when CO is dissolved in the xenon, the dinitrogen complex may be converted to CpRh(C 2 H 4 )CO by brief photolysis (Cp  η 5 -C 5 H 5 ).

Photolysis of group 6 metal carbonyl alkene complexes in liquefied noble gases: i.r. evidence for very labile dinitrogen and dihydrogen adducts

Broad-band u.v. photolysis of [(trans-cyclo-octene)xM(CO)6 –x](x= 1 or 2, M = Cr or W) in liquid Kr or Xe solution doped with N2 or D2 leads to formation of thermally labile complexes in which an alkene and either N2 or D2 are co-ordinated to the same metal centre; in each case the presence of the alkene appears to weaken the M–N2 or M–D2 bond.

Ethene complexes of chromium; the generation and characterisation of [Cr(CO)5(C2H4)] and cis- and trans-[Cr(CO)4(C2H4)2] and their relative stabilities in liquid xenon solution

The unstable complexes [Cr(CO)5(C2H4)] and cis- and trans-[Cr(CO)4(C2H4)2] are generated by photolysis of [Cr(CO)6] in C2H4-doped liquid xenon and are characterised by i.r. spectroscopy; these complexes are all unstable, even at -78 °C with the cis-[Cr(CO)4(C2H4)2] complex being the least stable, decomposing with an enthalpy of activation, ΔH‡, of ∼60 kJ mol–1.