Trans labilization

The first step in the reaction of ran,.s-[Fe(salpn)(H20)2l+, salpn=A(A7v-propylene-l,2-bis-salicylidiniminate, with sulfur(IV) is the formation of [Fe(S03)(salpn)(H20)], with the pH-rate profile showing greater trans-labilization by hydroxide than by water, in that traras-[Fe(salpn) (H20)2]+, reacts 10 times less rapidly than traras-[Fe(salpn)(OH)(H20)]. A limiting dissociative (D) mechanism is proposed for reaction of the latter formation of the sulfito complex is followed by a slow intermolecu-lar redox reaction (346). A similar situation prevails for the analogous irans-[Fe(salen)(H20)2]+/sulfur(IV) system (347). 

The syntheses, structures and properties of wide varieties of metal nitrosyl complexes have been well documented [4, 5, 20-23]. However, the bulk of the complexes reviewed previously are of academic interest and only a few of these metal nitrosyl complexes have been considered as biologically effective NO donors. It was observed that the metal nitrosyls with significant NO+ character are subject to attack from a variety of nucleophiles and have hypertensive properties. This could be due to the strong trans- labilizing effect of NO. In contrast, the metal nitrosyl compounds with the general formula [M(CN)5NO]n, where the NO ligand was either neutral (for M = Co) or anionic (for M = Cr) showed no vasodilatory effect [24]. 

Some kinetic data on the trans effect are now available. More detailed systematic studies are needed, but quantitative information does provide the magnitude of the trans labilizing ability of various ligands for substitution reactions in these systems. For complexes of the type [PtNH3LCl2]", where the leaving Cl" is trans to L, the trans effect order of L is approximately (28),... 

The large trans labilizing ability of H is explained on the basis of the polarization theory, whereas the x-bonding theory is involked to account for the large effect of PEt8. 

Furthermore, the heterolysis reaction is catalyzed by the addition of anions (organic and inorganic oxy anions, e.g., acetate) (44,107, 110,111). Comparing the acetate effect in the presence of different chelating ligands ([15]aneN4 and nta) to those of the aquated system led to the conclusion that the oxy anions have to occupy the trans position to the R group in order to labilize the M-R bond (trans labilization effect) and thus catalyze the heterolytic decomposition (44,107,110,111). 

In the triazenido complex, there is a slight trans labilizing effect of the hydride over the carbonyl, as reflected in the trans Ru-N distances. This effect is much more pronounced in the amidinato complex because of the absence of disorder and perhaps because the four-membered ring has opened up so that atom N(2) is more nearly trans to the hydride ligand. 

The isomers cis- and trans-[Cr(CN)2(NH3)4]+ as well as m-[Cr(CN)(NH3)4DMS0](G04)2 have also been synthesized by the anation reactions shown in Scheme 29.325 While anation in DMSO is accompanied by stereochemical change, the reaction in H20 is stereoretentive. The dicyano complexes undergo H+-assisted thermal aquation, involving the successive loss of the CN- ligands. The trans complex is about ten-fold more reactive in the first step than the cis, an observation attributed to the trans labilizing effect of CN-. 

For [cis-Pt(NH3)2(N-het)Cl]+ compounds an ammonia-loss pathway, as a result of the trans labilizing effect of a N-het, to achieve didentate binding to DNA has been considered as a possible mechanism of action... 

The chemistry of the coordinated nitrido group is relatively sparse. The strong trans-labilizing influence of the nitrido group, and the effect this has on the facile exchange and substitution of the trans ligand and the structure of nitrido complexes has been discussed (132). 

To study this process, a very suitable compound appeared to be [Pt(dien)Cl]Cl (dien = 1, 5-diamino-3-azapentane Fig. 3). This compound is readily available, forms relatively stable complexes with S-donor ligands, has only one substitution step, and is not complicated by (often occurring) subsequent reactions caused by the trans-labilizing effects of S-donor ligands. 

Os(VI) complexes of type [Os(VI)NX5]2- or [0s(VI)NX4H20]-(X = Cl, Br, CN, ox) are made by the action of the appropriate acid HX on potassium osmiamate (6, 11). The tendency to lose the ligand trans to the nitride and to substitute a water molecule is some evidence of a trans labilizing effect of jr-donor ligands. X-ray studies show a distorted octahedral structure with the osmium atom slightly out of the plane of the equatorial X atoms towards the nitride (1). The Os=N stretching frequency is around 1080 cm-1 (11) in such complexes. 

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