Five-Coordinate Platinum(IV) Complexes

Reversible C–C bond formation at a triply cyclometallated platinum(iv) centre

“…A much more efficient, and more readily understandable, method of inducing the reductive coupling of the phenyl pyridine with the benzyl phosphine is the addition of one equivalent of silver tetrafluoroborate to 2(t) . The silver salt removes the chloride, generating a reactive five-coordinate intermediate, 10d , 21 and a reductive coupling reaction between two of the carbon bonded groups takes place. The rapid reductive elimination from an unsaturated intermediate has been rationalised theoretically, 22 with the argument hinging on the fact that the coupling process results in the population of a metal orbital that is only non-bonding in the five-coordinate complex, but anti-bonding in a six-coordinate complex.…”

Oxidation of dimethylplatinum(II) complexes with malonyl peroxide derivatives

“…The equilibrium constants for formation of the oxalate complex 5 and phthalate complex 6 were 39 and 0.49, respectively. The experiments clearly show that the Pt−O bonds in 2a, 5, and 6 are easily cleaved (note that substitution at platinum(IV) usually occurs by a dissociative mechanism involving a square pyramidal intermediate) 54,55 and also support the expected stability series as a function of chelate ring size, with the five-membered Pt(oxalate) most favoured and the seven-membered Pt(phthalate) least favoured, although the difference between phthalate and cyclobutane malonate is small. 35,36 The sequence of pK a values of the dicarboxylic acids follows the sequence oxalic acid (pK a 1.2), phthalic acid (pK a 2.9), and cyclobutane malonic acid (pK a 3.13), correlating with the electronwithdrawing ability of the group attached to the carboxylic acid.…”

The mechanism of oxidative addition of iodine to a dimethylplatinum(II) complex

“…For this to occur, complex 2a must isomerize slowly during the crystallization process, which typically took several weeks at room temperature, with the co-crystal being less soluble than 2a. The equilibration is presumed to take place by way of the intermediates 5 and 7 (Scheme 4) [25,26]. The solutions containing a high proportion of 2b only slowly converted back towards the equilibrium mixture containing mostly 2a over a period of weeks, so it is clear that 2a is the major product of kinetic control and that it is also the more stable isomer in solution in solvents benzene and dichloromethane.…”

“…The stepwise, polar mechanism of Scheme 3 usually leads to trans oxidative addition [1e8,10e24] but, since likely 5-coordinate intermediates are stereochemically non-rigid [25,26], cis oxidative addition can be observed [13] or equilibration between trans and cis isomers can be observed if thermodynamically favorable [14,24e26]. There appear to be no well-established examples of concerted, non-polar, cis oxidative addition of halogens to platinum(II) complexes [1e4,13].…”