Five coordinate platinum

The first examples of five-coordinate platinum(II) complexes of the type [Pt(PR3)L]2+ (L = tris(2-(diphenylphosphino)ethyl)phosphine R = Et, OMe, OEt) (104) containing only P-donor atoms have been prepared by the reaction of [PtClL]+ with an appropriate monodentate tertiary phosphine or phosphite ligand.284 Triaryl phosphines and phosphites do not react with the precursor complex, even at elevated temperatures, most probably due to the considerable steric interactions that would occur upon the approach of the P-donor ligand to the platinum(II) center. 

The reactivity of the closely related system TpMe2PtMeH2 toward electrophiles in arene solvents has also been reported recently (68). The boron-based Lewis acid B(C6F5)3 induced elimination of methane and formation of an aryl(dihydrido) platinum(IV) complex via arene C-H activation (Scheme 17, A -> C). The active acid may be either B(C6F5)3 or alternatively a proton generated from B(C6F5)3 and trace water. It was proposed that the acid coordinates to a pyrazole nitrogen (shown in Scheme 17, B) forming an intermediate five-coordinate platinum(IV) complex, which readily eliminates methane. 

Both four- and five-coordinate platinum(II) complexes can be prepared with phosphole ligands L (119).1328 The complexes /rans-PtX2L2 (X = Cl, Br, I R = Me, Bun, Bu, Ph, CH2Ph R = H, Me) are non-electrolytes in methanol solution. The formation of the five-coordinate adduct (equation 411) can be analyzed in terms of intra- and inter-molecular equilibria of the pentacoordinate species PtX2L3. The formation of PtX2L3 is enthalpy favored and entropy disfavored. 

The method of preparation of the five-coordinate platinum(II) complexes of the above diamines is identical to the one used in Section 19-A. All the compounds can be purified by recrystallization from a dichloromethane/methanol mixture. The yields range 65-85% (see Table I). 

Eekl U, Kaminsky W, Goldberg KI (2001) A stable five-coordinate platinum(IV) alkyl complex. J Am Chem Soc 123 6423-6424... 

Five-coordinate alkyl Pt(rv) complexes have been proposed as short-lived intermediates in platinum-catalyzed alkane functionalization cycles. Hence, interest in preparing suitable complexes to enable their chemistry to be studied has arisen.Novel five-coordinate platinum(iv) alkyl complexes with a variety of /3-diiminate ligands have been prepared and have been shown to be useful precursors to unsaturated Pt(ll) species for alkane. Stoichiometric alkane dehydrogenation was observed using either a five-coordinate Pt(iv) species or an olefin hydride complex. Mechanistic proposals were based on NMR spectroscopic measurements and, in one case, on X-ray crystallographic characterization of a product. Kinetic details were not reported in this communication, but the systems hold potential promise, and conversion to a catalytic system may be feasible upon further investigation. 

A variety of compounds with metal metal bonds have been prepared. Mercurous compounds which contain the [Hg-Hg] " unit have long been known. More recent examples include compounds in which carbon monoxide is a ligand such as (Co)sMn Re(CO)s and (C6H5)3PAu-Co(CO)4. A number of complexes with transition metal tin bonds have been prepared. One of these, [Pt(SnCl3)5], contains five-coordinated platinum(ll) and is an excellent hydrogenation catalyst. 

Proposed Five-Coordinate Platinum(IV) Intermediates 2.1 Reductive Elimination from Pt(IV)... 

As discussed earlier, five-coordinate species have been identified as key intermediates in the reactions of six-coordinate Pt(IV) and four-coordinate Pt(II) metal complexes. The recent isolation of model five-coordinate platinum(IV) complexes... 

Luedtke AT (2007) Synthesis, structure, and reactivity of five-coordinate platinum(IV) complexes. PhD Thesis, University of Washington, Seattle, WA... 

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