Thiocyanate electronic effects

Electronic Effects The first thiocyanate linkage isomers were isoluled after it was noted that the struc-... 

Numerous additional examples of linkage isomerization in solids have been reported.13 Most studies have focused on elucidating the relative roles played by steric and electronic effects in determining the bonding mode of the nitrite or thiocyanate ligand. Interestingly, the bonding mode of thiocyanate has been found to be subject to counter-ion control in the solid state.20... 

Electronic effects become apparent in the M—NCS/M—SCN linkage switches observed in a series of Pd11 (iso)thiocyanate complexes. Ligands positioned trans to the pseudohalide and that are suited to accept electron density from the metal into empty orbitals (backbonding) stabilize the Pd—NCS linkage isomer. However, this rationale is contradicted by the trend in Co—(NCS) bonding in a series of cobalt complexes (see ref. 204b for a review). 

The previous discussion has been concerned with thiocyanate complexes, but a similar situation obtains with selenocyanates [437, 438). Table XI includes the frequency ranges for different modes of seleno-cyanate coordination. Kharitonov et al. (439) obtained similar results for selenocyanate complexes as for thiocyanate complexes in their theoretical treatment outlined previously, and their conclusions are the same in both cases. Thus, frequency shifts can be due to electronic effects or to changes in the bonding mode. The relative contributions of the resonance forms of NCSe (Table X) are comparable to those of NCS so that integrated intensities can be used in a similar way, and with the same possible dangers. Table XII contains some infrared data... 

Fig. 12.46 Structures of four palladium complexes illustrating combined steric and electronic effects on bonding of the thiocyanate ligand. [From Palenik, G. J. Steffen, W. L. Mathew, M. Li, M. Meek, D. W. Inorg. Nucl. Chem. Leu. 1974. 10, 125-128. Reproduced with permission.]...

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. 

Much discussion on the coordination chemistry of the thiocyanate ligand centers on its function as an ambidentate ligand. For coordination to platinum both N-bonded and S-bonded thiocyanate ligands are found and the small energy difference between the two bonding modes leads to small effects such as steric and electronic factors, as well as solvent changes, causing conversion between S- and N-bonded thiocyanate. 

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