Thiocyanate steric effects

However, the energy difference between N- and S-bonded thiocyanate is very small and is influenced by an interplay of several factors steric effects, solvent and the counter-ion in ionic complexes. To illustrate the last point, in complexes [Pd[Et2N(CH2)2NH(CH2)2NH2]NCS]+, the PFg salt is N-bonded, as it is in the unsolvated BPhg salt. However, though the acetone solvate of the tetraphenylborate is N-bonded, the methanol solvate is S-bonded [126],... 

Subsequently it was shown that the P-Pd-P angles were essentially the same as in the corresponding chloride complexes (section 3.8.3) as a result, as the P—Pd—P angle increases, concomitant upon the increase in the length of the methylene chain, steric effects enforce N-bonded thiocyanate, which is less sterically demanding that the non-linear Pd-SCN linkage (favoured on HSAB considerations since Pd2+ is a soft acid and sulphur is a soft base). 

Nature of Cooperative Effects in Mixed-Ligand Thiocyanate Complexes Various explanations have been put forward to account for the effects of neutral ligands on the nature of the metal-thiocyanate bond TT-bonding, symbiosis, and antisymbiosis of hard and soft acids and bases as well as steric effects have all been discussed. In addition, directional... 

Electronic and steric effects have been compared and used to explain the mixed thiocyanate coordination when chelates with two different donor atoms are used (526), but it is now apparent that steric effects could be largely or even solely responsible. 

Most palladium (II) selenocyanate complexes have a Pd—Se bond, but the compound [Pd(Et4dien)(SeCN)]BPh4, isolated at low temperatures, isomerizes in a number of different solvents via a dissociative process, whereas [Pd(dien)(SeCN)]BPh4 shows no signs of such isomerization. Further, if [Pd(Et4dien)NCSe]BPh4 is isolated it reisomerizes to the Se-bonded form at room temperature in the solid state 153, 156). This behavior parallels that of the thiocyanate group under similar circumstances and provides evidence for a steric effect modified by the nature of the anion. 

M — N bonds in solvents of low dielectric constant,as shown in Figure 9-19(a). There are also compounds with both M-SCN (thiocyanate) and M-NCS (isothio-cyanato) [isothiocyanatothiocyanato(l-diphenylphosphino-3-dimethylaininopropane) palladium(II) Figure 9-19(b)]. M-NCS combinations are linear and M-SCN combinations are bent at the S atom in all thiocyanate complexes. This bend means that the M-SCN isomer has a larger steric effect, particularly if it can rotate about the M — S bond. 

An alternative preparative route to the unstable phosphorus thiocyanates uses McsSiCN in reaction with sulphenyl chlorides R R P(0)SC1, where R = R = Me3CCH20 or Me2CHO, and R = Me C, R = Ph. Because of steric effects, these products are more stable than those previously isolated. 

In the case of the linear Au(I) complexes, the coordination mode of the thiocyanate ligand reflects solely the electronic effect of the trans ligand, being free from its steric effect. The stronger the trans influence of L, the harder the remaining site becomes and, consequently, the greater the Au— NCS/Au—SCN ratio. 

A decisive solvent effect is also observed with other a,/ -epoxy ketones. Specifically, 3jS-hydroxy-16a,17a-epoxypregn-5-en-20-one and its acetate do not react with thiocyanic acid in ether or chloroform. However, the corresponding thiocyanatohydrins are formed by heating an acetic acid solution of the epoxide and potassium thiocyanate. As expected, the ring opening reaction is subject to steric hindrance. For example, 3j6-acetoxy-14f ,15f5-epoxy-5) -card-20(22)-enoIide is inert to thiocyanic acid in chloroform, whereas the 14a,15a-epoxide reacts readily under these conditions.Reactions of 14a,15a-epoxides in the cardenolide series yields isothiocyanatohydrins, e.g., (135), in addition to the normal thiocyanatohydrin, e.g., (134). 

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