Linear block metal compounds

There are many other shapes possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), [Hg(CH3)2] (5), which is a toxic compound formed by bacterial action on aqueous solutions of Hg2+ ions. Coordination numbers of up to 12 are found for members of the f block, but they are rare in the d block. One interesting type of d-metal compound in which the coordination number is formally only 2 but there are 10 links between the ligands and the central metal ion is ferrocene, dicyclopentadienyliron(O), [Fe(C5H5)2] (6). Ferrocene is an aptly named sandwich compound, with the two cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

Living radical polymerization (atom transfer radical pol5mierization) has been developed which allows for the controlled polymerization of acrylonitrile and comonomers to produce well defined linear homopolymer, statistical copolymers, block copolymers, and gradient copolymers (214-217). Well-defined diblock copolymers with a polystyrene and an acrylonitrile-styrene (or isoprene) copolymer sequence have been prepared (218,219). The stereospecific acrylonitrile polymers are made by solid-state urea clathrate polymerization (220) and organometallic compounds of alkali and alkaline-earth metals initiated polymerization (221). 

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