Uni- and Bivalent Heavy Transition Metal Ions

High-pressure NMR studies of solvent exchange processes at labile octahedral metal ions have been a valuable resource in the understanding of 

Steric crowding dramatically reduces the rate of ligand sustitution at square-planar complexes and such behavior has been described as peudo-octahedral . This reduction in rate is also observed for reactions of [Pd(pnp)Cl] (18), where AS is found to be in the range —100 to 150JK mol and AV —1 to — 13cm mol S supporting earlier proposals, based on hindered dien systems, that the mechanism still remains associative. 

The rates and equilibria for interconversion between successively substituted ethylenediamine-aqua-chloro-Pd(II) complexes in acidic solution 

Replacement of one of the cyclic dithiaether ligands in [Pd(dcto)2] (dtco = 1,5-dithiacyclooctane) by 2,2 -bipyridyl or phenanthroline in MeOH/water follows second-order kinetics (kypy = 20.0 0.2 s S A/f = 

4kcalmol- , A5+=-32 1 cal K mol ifcphe = 70 2M- s ) and are proposed to occur by an associative mechanism. When the mercaptomaleonitrile dianion (mnt ) is the entering ligand, the intermediate [Pd(dcto)(mnt)] is very rapidly formed (k 2x 10 M s ), which by comparison to the bipy and phen reactions supports the associative nature of the substitution this complex then reacts further to give [Pd(mnt)2] (k=14.5 0.2M- s- ). 

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