Platinum reactions

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA 

Professor Joseph Chatt was one of the truly outstanding inorganic chemists of the twentieth century. 

Immediately when chemists think of platinum(ii) complexes, they know it will almost certainly involve Chatt chemistry. He could rightly be called the Father of Platinum(ii) Coordination Chemistry in the UK, because the scientific literature abounds with his publications on important aspects of platinum(ii) chemistry. I do believe that his fundamental seminal research on these systems is second to none. 

In 1996, Coordination Chemistry Reviews (CCR) dedicated one of its issues to the memory of Joseph Chatt. I published an article in that issue entitled, Recollections of early studies on platinum(ii) complexes related to Chatt s contributions to coordination chemistry . For this reason, and because some authors will discuss platinum(ii) chemistry in other chapters, I will devote most of this paper to our research on the kinetics and mechanisms of platinum(ii) ligand displacement, and to the chemistry of platinum(iv) complexes. Since Joseph did little research on platinum(iv) chemistry, it seems appropriate that I review our work on it. Following this, I will briefly describe another area we worked on that interested Chatt although he was not involved in such research. This was that of making use of reaction mechanisms to help design the synthesis of metal complexes. 

2 Kinetics and Mechanisms of Ligand Substitution Reactions of Platinum(II) Complexes 

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Reactions of Pt(PPhj)2(R)X (R = CH3, C Hj X = Cl, Br, I) and methyl isocyanide 144) and analogous reactions of Pd(phos)2(CH3)I (phos = PPh3, PPhMc2) complexes with cyclohexyl isocyanide 169, 170) were reported about the same time. As might perhaps be anticipated, the platinum reactions were slower and one can isolate the intermediate species and observe their rearrangement to the inserted products [Eq. (8)]. The isolation of the... 

Additional work on alkyl platinum reactions has been completed which mostly supports the generality of these observed insertion reactions. Treichel and Wagner 153) have varied the choice of phosphine ligand utilizing the complexes Pt(phos)2(CH3)X (phos = PEtj, X = Br, I and phos = PPhMe2, X = I) both ionic intermediate and inserted products were obtained. 

At present, almost all sulfuric acid is made by the contact process, which has been in use since 1831. The first step is exothermic air oxidation of SO2 catalyzed by vanadium pentoxide (V2O5) or platinum (reaction 10.5). The yield of SO3 is limited on the first pass to some 60% because the temperature rises to 600 °C or more usually, three more passes over the catalyst are made, and the yield can be increased to 98%. The SO3 vapor is then absorbed into 100% H2SO4 (reaction 10.6), and water is added to the resulting mixture of disulfuric (H2S207) and sulfuric acids (known as oleum) until the H2S2O7 is all hydrolyzed to H2SO4 (reaction 10.7). This obviates the aerosol problem. 

Similarities between [Ru(bpy),]2+ (discussed in Chapter 13) and [Pt,(pop)J4 are apparent. Reactive excited states are produced in each when it is subjected to visible light. The excited state ruthenium cation, [Ru(bpy)3]" +, can catalytically convert water to hydrogen and oxygen. The excited slate platinum anion, [Pt,(pop)J 4-, can catalytically convert secondary alcohols to hydrogen and ketones. An important difference, however, is that the ruthenium excited stale species results from (he transfer of an electron from the metal to a bpy ligand, while in the platinum excited state species the two unpaired electrons are metal centered. As a consequence, platinum reactions can occur by inner sphere mechanisms (an axial coordination site is available), a mode of reaction rot readily available to the 18-clectron ruthenium complex.-03... 

In a review of explosions involving derivatives of gold, silver and platinum, reactions of ammonia with gold and silver compounds, and of hydrogen in presence of platinum compounds are emphasised. 

Palladium and platinum. Reactions of alkali metal acetylides, M2C2 (M = Na, K) with Pd or Pt (550 °C, in argon) gave black M2(Pd/Pt)C2,... 

Z. Xue, H. Thridandam, H.D. Kaesz, and R.F. Hicks, Organometallic Chemical Vapor Deposition of Platinum. Reaction Kinetics and Vapor Pressures of Precursors, Chemical Materials, Vol.4, 1992, pp.l62-166. 

Preparation of Products. A platinum reaction boat is carefully cleaned before each run by allowing it to remain immersed in fused potassium disulfate (pyrosulfate), K2S2O7, at least at red heat, for 15 minutes. A fume hood is necessary Upon removal from the disulfate, the platinum boat is immersed in boiling water for one-half hour in order to remove any traces of disulfate. Finally, the boat is rinsed well with distilled water and dried in a clean drying oven at 115°. Extreme care is taken not to touch or otherwise contaminate the boat. (This can be easily accomplished by using a clean white card to support the boat whenever it is necessary to transfer it. A stiff length of wire bent at one end to form a hook can be used to move and position the boat.) Samples that are used for the study of transport properties should be as pure as possible because these properties are influenced markedly by trace impurities. 

As explained previously, reactions of the type 6.3.3 show a great increase in rate on transfer from methanol to dimethylsulphoxide (in this case 10 -10 ) this increase is due to the great destabilisation of the chloride ion in dimethylsulphoxide which is not compensated by de-stabilisation of the large but tight trigonal-bipyramidal transition state. This great increase in rate is not shown in the transfer of the platinum reaction 6.3.2 into the dipolar aprotic solvent. Thus in the platinum case there must be stabilisation of the reactant complex or substantial destabilisation of the transition state, or a combination of both, to counteract the great increase of chloride ion activity on transfer to dimethylsulphoxide. 

When hydrochloric acid solutions of noble metal salts are being tested, the presence of silver salts need not be considered. Gold chloride can be removed by shaking the strong hydrochloric acid solution with ether or ethyl acetate. The gold-free aqueous solution should be taken to dryness to remove the excess hydrochloric acid before the test with / -nitroso-diphenylamine is tried. There is as yet no method for the removal of platinum which is suitable for the spot test technique. Since the identification limit of the platinum reaction with />-nitrosodiphenylamine is 10 y, the palladium test just given is not impaired by small amounts of platinum. 

Ersoy, D.A., McNallan, M.J., and Gogotsi, Y. Platinum reactions with carbon coatings produced by high temperature chlorination of silicon carbide. Journal of the Electrochemical Society 148, C774-C779, 2001. 

In certain Faradaic reactions, the product remains bound to the electrode surface. Examples include hydrogen atom plating on platinum (reaction 6) and oxide formation (reaction 13 as an example). These can be considered a logical extreme of slow... 

Table 2 Platinum reactions, potential listed in reference to standard hydrogen electrode [7]...

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