Platinum complexes acetylene

The protonation of zero-valent platinum-acetylene complexes has been discussed by several workers, and a mechanism proposed involving insertion of the acetylene into a hydroplatinum bond 287). The reaction... 

Mono-olefin and acetylene complexes of nickel, palladium and platinum... 

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

It was somewhat puzzling that while olefin compounds of platinum(II) were well established, acetylene complexes were virtually unknown. Acetylene gives red intractable materials on reaction with potassium chloroplatinate(II) in aqueous solution, presumably acetylides and dimethylacetylene does not react, in marked contrast to ethylene, which forms [PtCl3(C2H4)] . However, Gel man, Bukhovets, and Meilakh (44)... 

In structure (I) the platinum has effectively a coordination number of 3 with the alkyne occupying one coordination site, while in (II), with a coordination number for platinum of 4, acetylene behaves as a bidentate ligand. Preliminary X-ray data for (Ph3P)2Pt(C2Ph2) (16) favor structure (II) but do not completely eliminate structure (I). Bond character in Pt-acetylene complexes has also been discussed (17). 

Carbonatobis(triphenylphosphine)platinum(II) is useful for the preparation of dianionobis(triphenylphosphine)platinum(II) complexes as well as olefin, acetylene and carbonyl derivatives of platinum(0). The method can be used to prepare Pt[P(C6Hs)2CH3] jCCOa) and Pt[As(C6Hs)3] 2(C03). A procedure is available for the preparation of ds-[diacetatobis(diphenylphosphino)pla-tinum] (ds-[Pt(CH3C02)2[P(C6Hs)2l 2]). 

Recent literature describes the synthesis of vinyl esters in the presence of platinum metal complexes. Complexes which have proven particularly suitable in this context are those of ruthenium (eq. (15)), such as, for example, cyclooc-tadienylruthenium halides [36], ruthenium carbonyl complexes, and ruthenium acetate complexes [37]. A characteristic feature of these is their high selectivity with regard to acetylene, so that the production of acetylene polymers is reduced. 

Some time later, Rob Guy, whose chemical skills were much respected by the research team and who was also envied for his ability to charm ladies, managed to synthesise a range of acetylene complexes of platinum. Their infrared spectra indicated that bonding of the hydrocarbon to the metal was very similar to that in olefin complexes. Interestingly, it was observed that a-hydroxyacetylenes are chelated to platinum through interaction of oxygen lone-pair electrons with a vacant 6p orbital of the metal. 

Studies on the preparation and reactions of olefin complexes have been the subject of several recent reviews. In particular olefin and acetylene complexes of platinum and palladium have been reviewed by Hartley 0, characterised ole-fin-metal complexes by Quinn and Tsai ) and some acetylene-metal complexes by Greaves, Lock and Maitlis 3>. 

In olefin and acetylene complexes of transition metals, in contrast to those of silver(I) and possibly copper(I), the 7r-component of the co-ordinate bond is considered to predominate over the o-component. This has been inferred from the large shifts in nc=c on co-ordination of the unsaturated ligands. Olefins on co-ordination to platinum and palladium show a decrease in vc=c of 140 cm-1 and 120 cm-1 respectively acetylenes, on the other hand, show decrease of 250 cm-1 and 400-575 cm-1 on co-ordination to platinum(II) and platinum (0) respectively ls2h... 

The ff-cycloalkenylPt(II) complexes are less readily cleaved by xs HX give the corresponding olefin than are the complexes formed from acyclic acetylene platinum(0) complexes. 

The authors concluded that their NMR parameters suggested a con-tinum of bonding in platinum-olefin/acetylene complexes. The NMR parameters for this series of platinum-olefin/acetylene complexes are given in Table XLI. 

Miscellaneous.— The reaction between dioxygen platinum(ii) complexes and substituted acetylenes takes place via two discernible steps, according to reaction (9), where R = COaMe and R = cyclohexyl. There is a direct attack by the acetylene on the dioxygen ligand. Formation of u-peroxo- -hydroxo-complexes... 

The X-ray structural studies of Howard, Spencer and Stone have established the platinum tris (olefin) complexes, Pt(olefin)3 [31], do indeed have the anticipated planar arrangement. Furthermore, all the known PtLj (olefin) [32] and PtL2 (acetylene) complexes have the planar conformations shown in (10) and (11). 

Cationic platinum(II) acetylene complexes react with a variety of nucleophiles. Chisholm and Clark showed that the reaction of methanol with coordinated disubsti-tuted acetylenes (generated in situ) affords trans u-bonded vinyl ether complexes (Equation 11.35). The trans stereochemistry of the cr-vinyl group of the product suggests that the nucleophilic attack occurs external to the metal and does not involve prior coordination of methanol, Reger has shown that stable cationic iron-alkyne complexes undergo reaction with a wide variety of nucleophiles to give stable cr-alkenyl complexes resulting from trans attack of the nucleophile on the coordinated acetylene (Equation 11.36). A variety... 

In neutral and anionic platinum(II) complexes the alkyne may be displaced by thiourea, KI, KSCN, KNO2, and Na2S203. Acetylenes exhibit a relatively strong trans effect. In [PtCl3(RCCR)]", the chlorine atom which is trans to acetylene is weakly bonded. The v(Pt-Cl . J frequency is lower than that of v(Pt —Cl ) by 20cm Consequently, ligands that are trans to alkyne readily undergo substitution. 

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