Trans effect halide ligand

Polymerization was carried out in benzene in the presence of bis-(7r-allylnickel halides). The latter were prepared from nickel carbonyl and allyl halide (allyl bromide, crotyl chloride, bromide, or iodide etc.). The results of the polymerization runs are reported in Table I. The data indicate that all of the bis(7r-allylnickel halides) initiate by themselves the stereospecific butadiene polymerization yielding a polymer with 97-98% 1,4-units. The cis-l,4/trans-l,4 ratio depends on the halide in the dimeric r-allylnickel halide but not on the nature of allylic ligand. The case of bis(7r-crotylnickel halides) shows the effect of halide on microstructure, for whereas (C4H7NiCl)2 initiates cis- 1,4-polybutadiene formation, trans-1,4 polymers are produced by (C4H7NiI)2. The reactivity increase in the series Cl < Br < I. 

Poe and co-workers have extensively studied the thermodynamics, kinetics and the trans effect in the reactivity of such complexes.623-634 635 638,64Cl-650-653 The thermodynamics of the successive anations of [Rh(en)2X(H20)]"+ by halogens suggests that the Rh center is a marginally soft acid, and that its softness is increased by the coordination of a soft base (such as I- ) in a position trans to the reaction site a halide ligand weakens the metal-ligand bond trans to itself, and an I- causes a more dramatic weakening than a Br-, which has more effect than a coordinated q-.634,638-640 The spectrophotometrically obtained thermodynamic parameters are summarized in Table 38. 

Tin compounds have been most studied,173 especially those of the SnCl3 ion, by 119Sn nmr and Mossbauer spectra. The SnClj ligand has a high trans effect and trans influence but has a rather low nucleophilicity. Thus in contrast to N, P, and As donors, SnCl2 will not cleave halide bridges in compounds with, for example, Pt(ju-Cl)2Pt bridges. 

Despite the high trans effect of the hydrido ligand there are also many six-coordinate hydrido complexes of general formula [RhHX2(ZR3)3] (X = halide Z = P, As). These complexes have two isomeric forms designated a and fi and are shown in structures (35) and (36), respectively. They can... 

Table VIII, part B), to compare directly the cis and trans effects of one ligand X (Table VTII, part C), and to see the effect of successive replacement of H2O by NH3 (Table VIII, part D). Formation constants involving the halides are given in Table VIII, part E, and some constants involving other ligands in Table VIII, part F. 

A vast array of complex ions, predominantly with halide or nitrogen donor ligands, are water-soluble. Exchange and kinetic studies have been made with many of these because of interest in (a) trans-effects, especially with square Pt11, (h) differences in substitution mechanisms between the ions of the three transition-metal series, and (c) the unusually rapid electron-transfer processes with heavy-metal complex ions. 

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