Thiocyanate complexes electronic effects

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. 

Another possibility is to apply the double-double effect to the differentiation of outer-from inner-sphere complexes of lanthanides and actinides because both the magnitude and direction of the effect depend on the difference in the electron-donor ability between water in the aquoion and the ligand in the complex. Therefore, a substantial difference between outer- and inner-sphere complexes with respect to the double-double effect should exist. Research in this field, using lanthanide complexes with thiocyanates as an example, is being made in our laboratory. 

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