Thiocyanate bonding modes

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. 

The nature of the solvent may modify the mode of thiocyanate coordination of certain complexes in solution. For example, a study of the behaviour of [Pd(NCS)2(AsPh3)2] and its linkage isomer in a number of different solvents concludes that Pd—SCN bonding is promoted by solvents with high dielectric constants, whereas solvents with low dielectric constants result in a mixture of Pd—NCS, Pd—SCN and Pd—SCN—Pd bonding modes.78... 

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... 

The terminal N-bonding mode to Fe has also been found in the thiocyanate complexes of iron.269"272 The Fe—N—C bond angle (a) of 160.6° in FeL(NCS)2 (95) has been ascribed to the absence of -electron density on the Fe center for -bonding to the NCS fragment. This -overlap would favor canonical structure (84a) and accordingly linear bonding. 

The thiocyanate ligands in the mercury complexes are either terminal S or 1,3-// bridge bonded. An example containing both bonding modes is Hg(NCS)2(PCy3).335 Heterodinuclear complexes have been discussed in section (vi) above. 

Cyanatc, thiocyanate, and selenocyanate groups can be linked to tellurium via the chalcogen atom or via the nitrogen atom. The v(C—X) infrared absorption bands have been used to distinguish between these two bonding modes. The iso (N-bonded) organic cyanates, R-NCX, have v(C-X) at higher wave numbers than the normal (X-bonded) compounds, R-XCN. 

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... 

We can now account for the two SCN bonding modes. The Hg ion is mnch larger and more polarizable (softer) than the smaller, harder Zn. The softer end of the thiocyanate ion is the sulfur. Consequently, the mercury ion [Hg(SCN)4] is a soft-soft combination, and the zinc ion [Zn(NCS)4] is a hard-hard combination, consistent with the HSAB prediction. Other soft cations such as Pd and Pt + form thiocyanate complexes attached through the softer sulfur harder cations snch as Ni and Cu form N-bonded thiocyanates. Intermediate transition-metal ions can in some cases bond to either end of thiocyanate. For example, both [Co(NH3)5(SCN)] and [Co(NH3)5(NCS)] are known, forming an example of linkage isomers (Chapter 9). 

The difference in the charge densities on the N and S atoms is not very large. This may be the reason why the NCS anion exhibits a variety of coordination modes as shown in Fig. 33. There are a large number of metal complexes containing an N-bonded or S-bonded thiocyanate ion as a unidentate ligand. Various factors influencing the relative stabilities of these two bonding modes will be discussed in Section 4.3. 

Relative stabilities of the thiocyanate linkage isomers in solution depend upon the nature of the solvent. Thus, square-planar Pd(II) and Pt(II) complexes of type ML2(CNS)2 (L = PPh3, AsPh3, SbPh3, and other neutral ligands) adopt only the S-bonded mode in solvents such as DMF... 

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