Thiocyanate ligands, bonding

Linkage isomerism This is a special type of structural isomerism in which the differences arise from a particular ligand which may coordinate to a metal ion in more than one way. In Table 1-3 we indicated that a ligand such as thiocyanate could bond to a metal through either the nitrogen or the sulfur atom, and the complex ions [Co(NH3)5(ACS)]2+ and [Co(NH3)5(5CN)]2+ are related as linkage isomers. 

The products of reduction of Fe(NCS)2+ by Cr2+ in aqueous solution containing thiocyanate ions (equation 36), include the isomers of [Cr(NCS)(SCN)(H20)4]+, which contains both bland S-bonded thiocyanate ligands.641 These isomers undergo spontaneous decomposition by parallel aquation (loss of the S-thiocyanato ligand) and isomerization (Cr—SCN— Cr—NCS) reactions. Details of the solution visible spectra of these isomers as well as those of similar chromium(III) complexes for comparison are listed in Table 68. From the compiled data it is apparent that the S-bonded thiocyanate ligand lies very close to Br- in the spectrochemical series while the N-bonded form lies between Nj and NO. 

The reaction of [MoCLt(NCMe)2] with K(SCN) in MeCN solution yields [Mo(NCS)6]2- and this anion has been isolated as its [NBu4]+ and [AsPh4]+ salts.188 Vibrational spectroscopy has indicated N-bonded rather than S-bonded thiocyanate ligands. The adduct [Mo(NCS)4(bipy)] has also been obtained.189... 

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 anion can play a part in determining the mode of bonding in the solid state. Thus the complex cation [Pd(CNS)(Et4dien)]+ has an N-bonded thiocyanate ligand in its most stable form when SCN- is the counterion,73 but when PF or BPhiT are the counterions the stable isomer in the solid state has S-bonded thiocyanate ligands.75,76... 

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 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. 

Fig. 6. Structure of the bis(thiocyanato) complex ppyAu(SCN)2 142, showing the presence of N-bonded (trans to C) and S-bonded (trans to N) thiocyanate ligands.

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