Five-coordinate cations

The precatalysts, Pt(IV) or Pt(II) salts, were found to be reduced in a series of steps by excess P(CH20H)5 to give Pt[P(CH20H)5]4, which in aqueous solution exists in equilibrium with the five-coordinate cationic hydride [PtL4H][OH] [L = P(CH20H)3]. Since reaction mixtures are basic [rationalized by the formation of hemiacetals from P(CH20H)3 and formaldehyde], the major Pt species present during catalysis is ze-... 

With P-donors a variety of different structures are formed. Two complexes (359a,b) with a five-coordinate square pyramidal geometry and one S-donor in the apical position are reported.934,933 In a related complex (360) the nickel center reveals a distorted square planar geometry, because one xanthate ligand switched to a monodentate coordination mode. 6 By reaction of [NiL2] complexes with Ph2P(CH2)2P(Ph)(CH2)2PPh2, compound (362) is formed, which consists of a five-coordinate cation and a hexacoordinate anion. 7... 

Chelating ligands have a much lower propensity for dissociation. Yet detailed kinetic studies on similar systems with bidentate phosphine ligands, /ac-(L2)PtMe3X (L2 = dppe (bis(diphenylphosphino)ethane), dppbz (bis(diphenylphosphinobenzene) X = I, OAc, OPh) also showed that ligand dissociation was required prior to any C-C coupling (48-51). In this case, however, the X- group rather than the phosphine was lost to form a five-coordinate intermediate, as shown in Scheme 11. A competitive C-X reductive elimination also occurs from these complexes and involves the same five-coordinate cation (Section V. A). 

Strong support for the mechanism shown in Scheme 48 was presented. Dissociation of OR- to form a five-coordinate cation occurs prior to reductive coupling. C-C reductive elimination occurs in an intramolecular fashion from this cation. In contrast, C-0 reductive elimination occurs by nucleophilic attack of the OR- on the Pt(IV)-methyl group of the five-coordinate cation. 

In addition to neutral five-coordinate aluminum alkyls, rare examples of five-coordinate cationic compounds have also been reported.101 The cationic compound [(PMDETA)AlMe2][Me2AlCl2] (PMDETA = pentamethyldiethylene triamine) (Figure 10(a)) was prepared by the reaction of Me2AlCl with PMDETA in toluene. The NMR showed... 

Figure 10 Five-coordinate cationic aluminum alkyls.

Two observations indicate the importance of steric hindrance in stabilizing a five-coordinate cationic intermediate (1) isomerization is more rapid for L = PMe2Ph than for L = PMe3 (see Table II) (2) rates... 

There are many complexes containing one bidentate and two monodentate neutral ligands. A preliminary crystal study has been made on one of them.1034 It may finally be noted that the oxidative addition of sundry reagents to rhodium(I) complexes is not an infallible preparative method for complexes of this type. Nitronium tetrafluoroborate, for example, gives only a five-coordinate cation (equation 230).1035... 

The complexes containing organonitrile ligands themselves undergo a variety of reactions (Scheme 44). Whilst the structures of the five-coordinate cations remain a matter for speculation, the six-coordinate cation [Rh(NO)(MeCN)3(PPh3)2]2+ has been shown to have the structure (138) by X-ray crystallography.1343... 

Rutile has a tetragonal lattice in which Ti ions occur coordinated to ions in slightly distorted octahedral sites, one-half of the octahedral sites being empty. The most stable crystal face appears to be the (110), and in Fig. 8.16 is shown the (110) surface that results from breaking the smallest number of cation-anion bonds (after Henrich, 1983, 1987). Two kinds of Ti cation are present on this surface, one with five ligands and one with all six ions, as in the bulk material. The local environment of the five-coordinated cation is similar to that of the surface cation in MgO. 

Zerovalent complexes of Ni, Pd, or Pt are protonated by HX (and by strong organic acids). In the case of Ni, five-coordinate cations may be formed. With Pt, protonation or addition of the HX unit may occur, depending on the nature of the anion ° ... 

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