Linear transition metal coordination

The first linear polymers containing [2]catenanes in their backbones (mechanically linked main chain polymers, architecture A) were based on the Sauvage transition metal-coordinated catenanes. Polycondensation reactions were carried out between a diacid and a catenane diol to give a poly[2]catenane with Mn (600000) and Mw (4200000) based on polystyrene standards (Figure 4). 

The complexes of copper(I) like those of silver(I) (p. 430), but unlike those of preceding transitions metals, tend to prefer a linear coordination of two ligands, i.e. X—Cu—X thus copper(I) chloride in aqueous ammonia gives the colourless [Cu(NH3)2] (readily oxidised in air to give blue [Cu (NH3)4(H20)2] copper(I) chloride in hydrochloric acid gives [CuClj], although [CuCl3] is also known. 

There are considerable numbers of the organogold compounds [3(b), 9,154], principally in the +1 and +3 oxidation states. Gold is unusual in transition metals in that, even in the +1 state, it has a marked preference for forming a-rather than zr-bonds, presumably related to the tendency of gold(I) to linear 2-coordination. 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

Simple monodentate NHCs are somewhat susceptible to dissociation when coordinated to early transition metals [6], so in most cases multidentate chelating hgands are employed in which the carbene is tethered to a strongly coordinating anchoring group. This is not universally the case however, and simple monodentate NHC complexes of Zr 1 (Fig. 4.1) have been studied [7]. The complexes were activated with MAO and tested for ethylene polymerisation, leading to moderate activities between 7 and 75 kg mol bar h for linear polyethylene. 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

By writing about complexes containing triple bonds between phosphorus and transition metals, one has to take into account the triple-bond character of phosphinidene complexes which are in a nearly linear coordination mode (type C) in contrast to the usual bent coordination mode D possessing typical double-bond features. Due to the additional r-donation bonding ability of the PR moiety to the metal atom in type C and the observed bond lengths, this type of complexes has to be included into the classes of metal-phosphorus triple bond compounds. Thus, at the end of this review will appear a chapter highlighting the appropriate compounds of type C. 

The simultaneous function of a transition metal atom as a lone-pair acceptor and as a cluster vertex atom is realized in [(77 -Ge9)Cu(ry -Geg)] (Fig. 5j), in which the two [Geg]" units are connected in an r/ - and an r/" -fashion to the same Cu ion. Transition metal bridges between Geg clusters are also found in various other coordination modes in [Ge9(/i -Au)3Geg] (Fig. 5k), three bridging Au atoms each are coordinated by two Ge atoms of two different Geg units in an almost linear way and entertain aurophUic interactions within the resulting triangle of Au" " ions two [GegR3] anions are coordinated via their triangular face to ( M = Zn, Cd,... 

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