Metals infinite polymers

The allyl lariat ether (Section 3.3) 3.132 forms complexes with both K+ and Ag+ (which are of similar ionic radius, Table 3.5). In the case of the K+ complex, as may be expected, the potassium ion is too large to fit snugly within the aza[15] crown-5 ring and lies somewhat above the donor atom plane. The exposed face of the metal atom is occupied by a PF6" anion. Similar coordination to the crown is exhibited by Ag+ but instead of an anion, the exposed face of the metal ion is coordinated to the allyl side chain of an adjacent molecule to give an infinite polymer in the solid state (Figure 3.91). 

When non-metallic compounds combine with one another they form compounds that are non-ionic and non-metallic, sometimes molecular (As4N4, P4Si0) and sometimes infinite polymers (SiS2, AS2S3, Si02). 

Both metals form bis-cyclopentadienyl compounds and numerous substituted derivatives. None of these have anything approaching a ferrocene-like structure since the M2+ ions have closed d-shells. Cp2Zn is an infinite polymer while Cp2Cd is a monomer with tj -CsHs groups. 

Interestingly, the heavier alkali analogues of LiSn(0-/-Bu)3 exist as onedimensional infinite polymers (242, 264) with a common Sn03M cage (where M = K. Rb, and Cs). By contrast, the M = Tl derivative exists as a monomer in the solid state. The polymeric structures for heavier alkali metal derivatives may be attributed to the higher coordination number requirements by the larger... 

In recent years there has been increased interest in mixed valent homonu-clear complexes which exhibit unusual electronic properties arising from the rapid electron transfer between the metal sites, such that the metal atoms are in the equivalent nonintegral oxidation state. These complexes may have interesting electrical and magnetic properties via a valence interchange mechanism if interaction occurs between isolated clusters or within an infinite polymer. To date only small isolated oligomers have been characterized. General reviews of the mixed valent complexes are available for the interested reader (198, 355, 455, 468, 534). 

In a similar manner, fluorides with the stoichiometry MF would have the capacity to make two fluoride bridges per metal atom, and those with the stoichiometry MFj could form three fluoride bridges per metal atom. These possibilities allow the formation of two-dimensional infinite polymer sheets and three-dimensional infinite polymer arrays, respectively, for the two stoichiometries. In addition to the polymeric possibilities, there are many examples of the excess of d electrons on each metal atom actually forming direct metal-metal bonds. This can lead to a delocalization of the metal d electrons that allows the compounds to be electrical conductors. 

The most investigated examples are to be formd in the precipitation of polyelectrolytes by metal ions. Here, networks are formed by the random crosslinking of linear polymer chains, and the theory requires some modification. The condition for the formation of an infinite network is that, on average, there must be more than two crosslinks per chain. Thus, the greater the length of a polymer chain the fewer crossUnks in the system as a whole are required. 

The theory of gelation (Flory, 1953,1974) has been summarized in Section 2.2.3. This theory regards gelation as the consequence of the random crosslinking of linear polymer chains to form an infinite three-dimensional network. The phenomenon is, of course, well illustrated by examples drawn from the gelation of polycarboxylic acids by metal ions. 

For the public, we made a video documentary of materials science education in which the National Academy of Sciences collaborated with WQED. This program was broadcast on public television as part of the Infinite Voyage series. The video on materials science education, called Miracles by Design , is useful for showing to students who are not sure what this field is all about. It covers the spectrum physics, chemistry, metals, ceramics, and polymers. An article in Business Week called The New Alchemy covered this field in some detail for the business community-... 

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