Platinum chain compounds

Its low specific gravity indicates that it is an open-chain compound, and from its easy reduction by sodium and alcohol, into dihydroaplotaxene, and by hydrogen and platinum black into normal heptadecane OjjHj,., it is evident that aplotaxene is a tetraolefinic normal chain hydrocarbon. 

In this way we come to class III complexes, i.e. complexes in which the two sites are indistinguishable and the element has a non-integral oxidation state (delocalized valence). Usually one divides this class in two subclasses. In class IIIA the delocalization of the valence electrons takes place within a cluster of equivalent metal ions only. An example is the [NbgCli2] ion in which there are six equivalent metal ions with oxidation state + 2.33. In class IIIB the delocalization is over the whole lattice. Examples are the linear chain compound K2Pt(CN)4.Bro.3o. 3H2O with a final oxidation state for platinum of 2.30, and three-dimensional bronzes like Na WOg. 

PtBrs (arsine) 2, which are octahedral complexes of platinum(IV) in which the olefinic side chains of the ligands have been fully brominated. An alternative and more realistic formulation of these compounds is PtBr4 (arsine Bre) 2- These same compounds are formed by reaction of an excess (> 3 equivalents) of bromine with the original platinum(II) compounds. 

Hubbard (13) elucidated a mathematical description of the change from one situation to another for the simplest case of a half-filled s band of a solid. His result is shown in Figure 11. For ratios of W/U greater than the critical value of 2/ /3 then a Fermi surface should be found and the system can be a metal. This critical point is associated with the Mott transition from metal to insulator. At smaller values than this parameter, then, a correlation, or Hubbard, gap exists and the system is an antiferromagnetic insulator. Both the undoped 2-1 -4 compound and the nickel analog of the one dimensional platinum chain are systems of this type. At the far left-hand side of Figure 11 we show pictorially the orbital occupancy of the upper and lower Hubbard bands. 

Among the less common oxidation states those of I and III have the most significance. Complexes of platinum(III) have been of interest for many years because of their intermediacy in substitution reactions of platinum(II) and (IV). More recently binuclear platinum(I) and (III) complexes have been isolated, and the chemistry of these new complexes will be of increasing interest in platinum chemistry. Platinum forms strong homometallic bonds giving rise to multimetallic chain compounds and cluster complexes. The increasing use of X-ray crystallography, and 31P and 19 PtNMR will allow systematic studies to be made on these multimetallic platinum complexes. 

Complexes of platinum having cyanide and non-stoichiometric quantities of halide ligand have been prepared. A review by Miller gives a broad coverage of this subject,285 and a book series Extended Linear Chain Compounds has a number of articles which are of direct interest and relevance to workers in this field.286 This section will only briefly cover the topic, and will emphasize the more recent work. 

Methyl-metal bond cleavage also occurred in the reactions of gold(I), gold(III), and platinum(II) compounds with thiols and selenols (207-209). The reactions of gold(I) and platinum(II) complexes were much faster than those of gold(III), due to the operation of a radical chain mechanism in the former cases. For the reactions with diphenylphosphine, however, the following order of reactivity was found (209) ... 

Compounds Containing Nickel, Palladium, and Platinum Chains... 

Inner Transition Metal Complexes With Chains of Metal Atoms 103 9.2.12.1. Compounds Containing Nickel, Palladium, and Platinum Chains 9.2.12.1.4. Halogen-Oxidized Complexes Containing 7t-Delocalized Ligands. 

Figure 9-5. Influence of temperature on lubrication by long-chain compounds. 1 Docosane on platinum. 2 Stearic acid on platinum. 3 Copper laurate on platinum. 4 1% lauric acid in paraffin oil on copper. 5 Sodium stearate on steel. Data by Bowden, Gregory and Tabor [17]. Adapted by permission from Natali, Vol. 156, No. 3952, pp. 99. Copyright 1945, Macmillian Journals Ltd.

The number of unsaturated bonds can be determined by a titration with a standard solution of bromine dissolved in glacial acetic acid. The sample is also dissolved in glacial acetic acid to which sodium and ammonium acetate are added, so as to give a final concentration of 0-5 and OT M respectively. The cathodic current of bromine is measured with a short circuit system using a rotating platinum indicator, and a calomel or chloranil reference electrode at the potential of the reference electrode. Several unsaturated open chain compounds and terpenes have been titrated in this way.( ) Other modifications of this technique have been used to determine the amount of unsaturation in fats in methanolic solutions. The standard solution of bromine is prepared in methanol, saturated with sodium bromide, and the methylester of stearic acid is added to suppress the maxima. If the methanolic solution is acidified, OT M potassium bromate with OT M sodium bromide can be used instead of bromine. The bromine generated in homogeneous solution can be used to analyse xylenol and cresol fractions of coal-tars. Other phenolic compounds may be similarly determined. The method was used for the determination of small concentrations of styrene... 

Figure 6.13 shows the tri-nuclear platinum(II) compound known as triplatin. As the name indicates, it has three platinum centers that can interact with DNA. This long chain compound seems to act by both intra- and interstrand cross-links that can be up to six base pairs apart. Unfortunately, it has large dose-limiting side effects. Future molecular modifications may ameliorate these side effects. 

A second approach has the c-DDP derivative actually included as part of the polymer backbone chain. In 1977 we initially synthesized the first poly(cis-dichlorodiammine platinum II) compound... 

It has a chain structure, with one unpaired electron per tetramer unit ( eff = 1.81 B) and can be regarded as a Pt Pt111 compound. ESR data suggest that the unpaired electron resides in a MO based on platinum 5dz2 orbitals directed along the tetramer chain. 

The wavenumbers of the MMCT transition between Pt(II) and Pt(IV) are related to the Pt(II)-Pt(IV) distance in the chain, as follows from a study of these compounds using different types of ligands. This relation is hnear [97]. ff the linear curve is extrapolated to a MMCT transition energy of zero, we obtain a distance equal to twice the Pt(IV)-ligand distance. This is the situation in which the Ugand, for example Cl or Br , is placed in between the two platinum ions, i.e. in which the difference between the platinum ions vanishes. 

Fig. 2. Relaxation modulus G(t) of a set of polydimethylsiloxane samples with increasing extent of crosslinking plotted against time of crosslinking. The linear PDMS chains (Mn 10 000, polydis-persity index 2) were endlinked with a four-functional silane crosslinker catalyzed by a platinum compound. Samples with different extent of reaction were prepared by poisoning the reaction at different times. The actual extent of reaction was not determined. Two of the samples are clearly before the gel point (LST) and two beyond. The third sample is very close to the gel point. Data of Chambon and Winter [5] evaluated by Baumgartel and Winter [8]...

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