Platinum linear-chain complexes

Also linear chain complexes such as Pt(etn)4Cl3 which is known colloquially as Wolf-fram s red (etn being an abbreviation for ethylamine) have been studied successfully by resonance Raman spectroscopy (Clark, 1984). As example, we show in Fig. 6.1-12 the resonance Raman spectrum of a related halogen-bridged linear-chain species, [Pt(pn)2] [Pt(pn)2Br2] [Cu3Br5]2 (Clark et al., 1980). Although this species contains a complicated copper bromine chain, the resonance Raman spectrum (Fig. 6.1-12) is completely dominated by bands attributed to the v fundamental and its overtones n U[ of the platinum-bromine chain. 

Results on a large number of linear-chain complexes of platinum are summarised in Table 11. Harmonic wavenumbers and anharmonicity constants have been determined in all cases. The normal coordinate seems to be related to the halogen movements involved in the proposed hopping process for the conductivity of these linear-chain mixed-valence complexes (95). The chain halogen atoms would need to move, on average, 0.54,0.38 and 0.22 A for chlorides, bromides and iodides, respectively, in order to reach the point midway between the two platinum atoms, i.e. to the situation of a platinum (III) chain. These values only differ by a factor of about two from the root-mean-square amplitudes of vibration of Vi in the Vj = 16 states these are calculated (91) to be 0.22 A for X = Cl (wi = 319.5 cm-i) and 0.20 A for X = Br (cji = 179.6 cm ). These distance changes are related to the shift in the equilibrium... 

Mixed-valence linear-chain complexes of palladium display very similar features to those of platinum (97). Indeed, the great intensity of the t j progression in all these cases means that resonance Raman spectroscopy provides a sensitive and rapid means for detecting the formation of new linear-chain complexes of this sort. 

Table 11. Resonance Raman data on mixed-valence linear-chain complexes of platinum...

ONE-DIMENSIONAL LINEAR-CHAIN COMPLEXES OF PLATINUM, PALLADIUM AND NICKEL... 

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. 

Halide bridged nickel(II)/platinum(IV) and palladium(II)/platinum(IV) complexes with onedimensional linear-chain structures have been studied extensively.556-559 These are analogous to the linear-chain platinum(II)/platinum(IV) systems described in Section 6.5.13 and have similar broad and intense intervalence bands.556-559 However, they have weaker metal-metal interactions resulting in more localized valences than the all platinum analogues.556-559... 

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. 

Resonance Raman spectroscopy, well known to be a sensitive probe of the nature of charge-transfer excited states, is now established to be a sensitive probe of intervalence states. In particular, onedimensional systems prove to be very amenable to study, and the results on a variety of linear-chain platinum complexes of both the Wolffram s red sort as well as the pop sort (pop = H2P2O52 ) are outlined. Brief mention is made of the application of resonance Raman spectroscopy to the study of electrochemically generated species. 

As mentioned earlier, palladium, rhodium, and platinum catalysts lead to superior regioselectivities because they work under milder reaction conditions (20-80 °C, 0.1-1 MPa CO) [11], e.g., bimetallic catalysts based on tin(II) chloride and either platinum or palladium complexes afford linear esters in up to 98 % selectivity [12]. In addition, catalyst systems with preference for branched isomers are known. A recent example employed palladium acetate immobilized on montmorillonite in the presence of triphenylphosphine and an acid promoter for the hydroesterification of aryl olefins (eq. (3)). The reaction is totally regiospecific for the branched isomer of aromatic olefins, while aliphatic olefins afford branched chain esters only regioselectively with n/i = 1 3 [13]. 

The bis(oxalato)platinate(II) complexes based on [Pt(C204)2] , forms another family of linear chain systems, and the partially oxidised complex series, Cx[Pt(C204)2]-n(H20) (C = K, Rb, Mg " ", Co, Ni, Zn ), was reported.According to elemental analysis and X-ray diffuse scattering measurements, the formal oxidation state of platinum in this series is estimated in the range of -1-2.19 to -f2.38. The average Pt-Pt distance is 2.84-2.89 A and the conductivity at room temperature is 10 -10 S cm , depending on the associated cation. 

Paramagnetic platinum(i) species are unexpectedly formed on electrochemical reduction of [MPtLaM] [L=isonitrile, M = CrCCOlaCp, Mo(CO)3Cp, W(CO)3Cp, Mn(CO)6, Fe(CO)3(NO), Co(CO>4]. Complexes containing linear chains of iron and molybdenum/tungsten atoms linked by AsMca-groups have been reported and [ Pd(l5(Me)3i 6H4) aO -Cl)CM-M)] [M=Mo(CO)3Cp, Co-(CO>4] has been shown to have a bent arrangement of metal atoms. ... 

In the early stage of the development of molecular conductors based on metal complexes, partially oxidized tetracyanoplatinate salts (for example, KCP K2 [Pt(CN)4]Br0.30-3H2O) and related materials were intensively studied [6], In this system, the square-planar platinum complexes are stacked to form a linear Pt-atom chain. The conduction band originates from the overlap of 5dz2 orbitals of the central platinum atom and exhibits the one-dimensional character. 

Linear polycarbosilanes and polycarbosiloxanes-especially those containing arylene units in the chain-have specific physico-chemical properties which can be applicable in heat-resistant materials [29-31]. Phenylene-silylene-ethylene-polymers, which may serve as potential substrates for applications as membrane materials are usually obtained in the presence of platinum catalysts [32], although other transihon-metal complexes have also been tested in this process. 

Figure 2.6 Chloro-bridged linear platinum chains dispersed as aggregates of a poly ion complex with a sulfonate amphiphile.

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