Non-chelate Complexes

The following two sections are concerned mainly with inorganic complexes of palladium and platinum. Closely related organometallic compounds are considered on pages 618 and 624. 

Complexes of Palladium(ll) and Platiniun(ll).—Non-chelate Complexes. Accurate redeterminations of the structures of potassium tetrachloro-platinate K2[PtCl4] and the analogous palladium complex confirm that the [MCU3 - anions are exactly square-planar. The mean Pd-Cl distance of 2.313(2) A is very slightly longer than the mean Pt-Cl distance of 2.308(2) A however, after correction for the effects of thermal motion, on the assumption that the chlorine atoms ride on the metal atoms, the values are equal 

The metal atom in rrans-bis(diaminomaleonitrile)dichloropaIladium(u), [PdCl8(CiH4N4)ij] (45), lies on a crystallographic centre of symmetry. Only one of the amino nitrogen atoms of each diamine ligand is co-ordinated to the metal. The Pd-Cl distance of 2.309(6) A is normal (Table 3). The Pd-N distance of 1.94(2) A is rather short most comparable Pd-N distances lie in the range 1.99—2.04 A. The central C-C distance is 1.51(4) A, 

Only one report has appeared on five-co-ordination in palladium(ii) and platinum(ii) complexes. In a preliminary note three compounds of the type [M(phos)3Bra], where M = Pd or Pt and phos is a 5-alkyl-5fl-dibenzophosphole ligand (28), are said to involve square-pyramidal coordination about the metal atoms, with all the co-ordinated phosphorus atoms in basal sites. In each molecule two of the planar phosphole ligands are mutually parallel, as was found in the analogous nickel(ii) complexes. No further details are given. 

Chelate Complexes. In the analysis of c/s-dichIoro-( wcso-2,3-diamino-butane)palladium(ii) (46) all the hydrogen atoms were located staggered conformations are found at the exocyclic C-C bonds, and the chelate 

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Ion-association complexes may be classified into three types non-chelated complexes chelated complexes oxonium systems. 

In order to determine the stability of the non-chelated complexes 14a and 14b at finite temperatures, we have performed unconstrained MD simulations (at 300K and BOOK) for the respective Pd- and Ni-based systems. The results showed that for both metals the geometries oscillate in the vicinity of the corresponding local minima. No spontaneous isomerization towards chelated structures was observed, nor towards any other isomers. Thus, it can be concluded that the non-chelated isomers are stable on their free-energy surfaces, and separated from other isomers by a non-negligible barriers. 

Finally, disilanes give oxidative addition products with chelating Pd phosphine complexes which, unlike non-chelating complexes, are thermally stable.81... 

There is evidence that, in solution, A1 and Cr complexes can exist in several varieties with both chelate and non-chelate complexing. 

Table 3 Selected metal-ligand distancesjK in square-planar non-chelate complexes of palladium u) and platinum(u)...

The value of stabihty constant for an ammonia complex with Ni(II) is 7.98, whereas with ethylenediamine (en) is 18.2. The differences in entropy between chelate and non-chelate complex reactions essentially cause this change in the values of stabihty constants. The formation of chelate results in greater disorder as it involves the formation of a larger number of free particles in the products, while there is no change in the number of particles during the formation of analogous non-chelated complexes. 

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