Associative mechanism from volumes

The replacement of X by pyridine in the complex PtX(dien)+ to give Pt(dien)py2+ (equation 549) has been studied under controlled conditions with a range of leaving groups X. These data are shown in Table is.1 87-1988 From these data the leaving group order is NOj > H20 > Cl- > Br > I- > N3 > SCN- > NO2 > CN-. Reactions such as these must be carried out under thermal conditions for accurate comparison since photoaquation can occur, albeit with a rather low quantum yield.1989 The volumes of activation of these reactions (equation 550) are all negative. An associative mechanism is proposed for the nucleophilic dependent path, but for the nucleophile independent pathway both associative and dissociative mechanisms need to be considered.1990... 

The second-order term in the rate laws for reactions of low-spin iron(II) diimine complexes with such nucleophiles as hydroxide and cyanide ions has been established as arising from a bimolecular reaction between complex and nucleophile.182 Activation volumes that were obtained for reactions of CN and OH with Fc(phcn)2 1 and Fe(bpy)3 + were in the range of +19.7 to +21.5cm3mol-1.183 Because these observations were not readily accounted for by an associative mechanism, a mechanism analogous to the Eigen-Wilkins mechanism of complex formation was introduced in which dissociative activation dominates in determining the observed activation volumes. However, subsequently it was shown that solvation... 

Energy can be converted from one form to another, but it cannot be created or destroyed (first law of thermodynamics). In chemistry we are concerned mainly with thermal energy, electrical energy, and mechanical energy, which is usually associated with pressure-volume work. 

It is convenient for a simplified analysis of activation volumes to consider octahedral complexes as essentially incompressible spherical species with a characteristic average radius. It has been assumed that a five coordinate intermediate (MLj) arising from the dissociative release of a neutral ligand (L) will occupy the same volume as its six-coordinate precursor MLg. The volume of [NiCNHj) ] (138 cm mof ) is identical to that calculated for the hypothetical [Ni(NH3)5] cation. For the analogous cobalt(III) system, the intermediate [Co(NH3)5] has been estimated to have the same volume as [Co(NH3)g] (55 cm mof ). In the case of an associative mechanism the seven coordinate intermediate (ML7) is believed to occupy the same intrinsic volume as its precursor MLg. 

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