Platinum complexes bond enthalpies

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

The Pt—O bond enthalpies for a series of amino acid complexes of platinum(II) fall in the range 322-397 kJ mol-1.15 1... 

Formation constants for silver(I)-olefin complexes have been obtained in aqueous solution by potentiometric methods by Hartley and Venanzi 45>. The ligands allylammonium perchlorate, but-2-enyl ammonium perchlorate, allyl alcohol and but-2-en-l-ol, were shown to have affinities for silver(I) comparable with acetate and fluoride ions the complexes are much weaker than the corresponding platinum(II) complexes, possibly due to the non-directional characteristic of the silver component of the a-bond (5s-orbital). The small difference between the formation constants for the allylammonium and but-2-enylammonium complexes and the larger difference between the alcohol complexes was assumed to be an enthalpy effect, arising from an electrostatic repulsion in the case of the unsaturated ammonium cations. 

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