The ambidentate thiocyanate ligand. Niobium-93 nuclear magnetic resonance detection of (thiocyanato)- and (isothiocyanato)niobium(V) complexes

Preparation and Spectroscopic Investigations of Mixed Octahedral Complexes and Clusters

“…The optical electronegativities of NCS ≈ 2.8 and SCN ≈ 2.6 have been approximated from electronic spectra of mixed linkage isomers of halogenothiocyanatoosmates(IV). , On this basis, a not unexpected, but interesting result arises from a comparison of the metal shifts in the two series, [M(NCS) n (SCN) 6 - n ] z - , with M = 93 Nb and 103 Rh. In full confidence of the correct assignment in the Nb system, a 93 Nb high-field shift from [Nb(SCN) 6 ] - at −780 ppm to [Nb(NCS) 6 ] - at −1342 ppm is observed, while 103 Rh of [Rh(SCN) 6 ] 3- resonates at highest field with a downfield shift as the number of N-bonded ligands increases . With respect to electronegativity and polarizability, 93 Nb exhibits inverse behavior in agreement with its already discussed IHD, while 103 Rh exhibits NED and NPD in agreement with its already discussed NHD (compare with section II.D.3).…”

“…From the 65 possible individual species, 16 have been identified and assigned by the pairwise additivity method, with interaction terms optimized by linear regression analysis. As a further result of prime importance, the existence of the series, [Nb(NCS) n -(SCN) 6-n ] -, n ) 0, 2, 4, 5, has been demonstrated stringently 246 (see section III.C). The octahedral series, [NbCl n Br 6-n ] -and [NbCl n -Br 5-n (CH 3 CN)], have been studied by 93 Nb CW NMR spectroscopy at 1.4 T, and the pairwise additivity model has been applied for the first time to octahedral complexes.…”

“…Well-established evidence of mixed complexes, [Nb(NCS) n (SCN) 6 - n ] - , n = 0, 2, 4, 5, 6, including the cis/trans isomers for n = 2 and the trans isomer for n = 4, has been given by 93 Nb NMR spectroscopy in combination with the pairwise additivity method ,,,, which has been considered until recently 260 as a purely S-bonded species.…”

“…245 In a reinvestigation of these reactions, it was not possible to synthesize [Nb(NCS) 7 ] 2using very pure (NbCl 5 ) 2 with KSCN in rigorously dried CH 3 CN under an N 2 inert atmosphere. 246 Furthermore, considerable deviations in the assignments in the [Nb(NCS) n Cl 6-n ] -series became apparent, which have been attributed to the different preparation techniques. 190b As the most essential result, thiocyanate has been found to behave as an ambidentate ligand in this series.…”

“…In this section mixed complexes will be discussed, which are formed exclusively by an ambidentate ligand. Complexes of this type are known with the thiocyanate ligand as [M(NCS) n (SCN) 6-n ] z- 74,246,[277][278][279][280][281][282][283][284][285][286] and with the selenocyanate ligand as found in the unique series [Os(NCSe) n (SeCN) 6-n ]. 287 By definition, linkage isomerism requires the existence of at least two coordination compounds of the same composition with ambidentate ligands, which are attached to the central atom by two different donor atoms or even by two chemically inequivalent atoms of the same kind.…”