Coordination of thiocyanate

Methods of preparing nitrito complexes of Ir(III) are shown in Scheme 3. Structural characterization of (NH4)3[Ir(N02)6l shows that each N02 ligand coordinates through the nitrogen atom (Ir-N = 2.060 A). The coordination of thiocyanate ligands (NCS ) is discussed in Section 6.6 in [Ir(NCS)6], the ligand exhibits Linkage Isomerism and... 

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

Besides complexes of thiosemicarbazones prepared from nitrogen heterocycles, iron(III) complexes of both 2-formylthiophene thiosemicarbazone, 26, and 2-acetylthiophene thiosemicarbazone, 27, have been isolated [155]. Low spin, distorted octahedral complexes of stoichiometry [Fe(26)2A2]A (A = Cl, Br, SCN) were found to be 1 1 electrolytes in nitromethane. Low spin Fe(27)3A3 (A = Cl, Br, SCN) complexes were also isolated, but their insolubility in organic solvents did not allow molar conductivity measurements. Infrared speetra indicate coordination of both via the azomethine nitrogen and thione sulfur, but not the thiophene sulfur. The thiocyanate complexes have spectral bands at 2065, 770 and 470 cm consistent with N-bonded thiocyanato ligands, but v(FeCl) and v(FeBr) were not assigned due to the large number of bands found in the spectra of the two ligands. 

The nature of coordination of anions such as nitrate, perchlorate, and thiocyanate has been studied by both infrared and Raman techniques. In the case of anions, such as nitrate and perchlorate, the vibrational spectra indicate whether they are ionic or coordinated and if coordinated, whether they are unidentate, bidentate or bridging. In the case of thiocyanate, the vibrational spectra are useful in deciding the site of coordination. The change in the site symmetry of the anion upon coordination leads to changes in vibrational spectra of anions like perchlorate, nitrate, perrhenate and hexa-fluorophosphate. These changes in the vibrational spectra have been used to indicate the nature of coordination. 

The relatively scanty information available and the limited research effort devoted to the study of ligand reactions cannot be attributed to the relative youth of the experimental area, for the literature contains scattered observations dating back to the earliest possible time at which such reactions could be recognized. Indeed, Werner (68, 76) utilized a ligand reaction in his classic demonstration of the manner of attachment of thiocyanate to cobalt (III). In his view, the conversion of thiocyanate to ammonia within the coordination sphere could only mean that SCN is attached to cobalt through the nitrogen atom (Equation 1). 

The structure determination of Zn(en)(NCS)Cl reveals monomeric tetrahedral zinc coordinated to thiocyanate via nitrogen.179 A structure determination of Cd(en)(N02)2 reveals a polymer containing cadmium in a seven-coordinate pentagonal prismatic environment the en functions as a bridging ligand, as does one of the nitrites.180... 

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... 

Some five-coordinate Pd11 thiocyanate complexes have been reported. Both [Pd(SCN)(2,9-Me2phen)2][C104]79 and [Pd(NCS) As(o-C6H4AsMe2)3 ][SCN]80 have been characterized, but the mode of coordination is unspecified in [Pd(CNS)L2][SCN] [L = l,8-bis(dimethylarsino)naphthalene] 81 in the latter system it would appear that six-coordination occurs in solution. 

Particular interest attaches to the thiocyanate complexes since the mode of coordination of this ambidentate ligand is markedly influenced by the other ligands present 15 isomerization can be brought about in a number of cases by heating the solid complex or by dissolution in an appropriate solvent. The type of coordination found in a particular complex may in general only be rationalized by a consideration of both electronic and steric factors. As an example of the latter, frans-[Pd(NCS)2(PPh3)2] contains N-bonded, linear NCS groups, but in the... 

Much interest has centred upon the [Pd(CNS)2(L—L)] complexes, since the nature of the thiocyanate coordination is influenced by the particular ligand present. Thus, X-ray diffraction studies have shown that the mode of thiocyanate coordination in [Pd(CNS)2 Ph2P(CH2)nPPh2 ] changes from both S for n = 1 (dppm), through one S and one N for n = 2 (dppe), to both N for n = 3 (dppp).52 Linkage isomerism amongst these compounds has also been studied by... 

Only a few structures of thiocyanate complexes are known for the lanthanides and actinides, i.e. for Er,341 Th342 and U.343 They all contain terminal N-bonded NCS and have as an interesting aspect a high coordination number, e.g. as in [NEt4]4[Th(NCS)8].342... 

In order to accommodate the small, highly charged calcium ion, the cryptand adopts an unsymmetrical conformation in which two of the chains are pushed apart to allow coordination of a water molecule and the N N distance is reduced to 5.44 A (42). In the corresponding barium complex, the structural unit consists of two cryptates, two water molecules, and four thiocyanate anions, two of which (9) are bound through nitrogen to the metal, (43). 

Other coordination modes in pseudohalide complexes are comparatively rare. Amongst them, we note the structure of complex [AgLSCN] 0.25L (L = bipy), in which two silver ions are simultaneously bound with N atoms from NCS groups [166], The pseudohalide complexes with simultaneous different kinds of coordination of the NCS group are also rare. In particular, the complex compound [CuL(HL)2][Cu(L)(SCN)(p-NCS)], where LH is 2-dimethylaminoethanol, contains within its coordination sphere both kinds S-terminal (108) and -bridge (109) thiocyanate groups [178],... 

At the same time, it is necessary to take into account that the approach described has a number of exceptions, related for example to the nature of other ligands forming pseudohalide complexes. A series of classic examples of inversion of the bond M — N —> M — S —> M — N have been reported [6,8,11,42-44,59] and are presented in Sec. 2.2.3.5. In this respect, we especially emphasize the capacity of other ligands for soft or hard metals, related with symbiotic [60] and anti-symbiotic [61] effects. Thus, Pearson [61] emphasized that soft ligands, which are placed in a trans position to SCN ion, contribute to N-binding of thiocyanate ions, and hard bases contribute to S-coordination of these ambidentate ligands. Metal oxidation number (Table 1.4) is important in this problem and it regulates soft hard properties of complex-formers. 

Anion sensing using visible-emitting lanthanide probes has proven successful (Tsukube et al., 2006) and this work is now being extended to Ybm probes, particularly for the detection of thiocyanate. The latter is the principal metabolite of cyanide anion and exists in human serum, saliva, and urine. The luminescent probe is a complex of hexadentate tetrakis(2-pyridylmethyl)ethylenediamine (tpen, see fig. 119) which bears two water molecules, [Yb(tpen)(H20)2](0tf)3. In absence of anion coordination, the 980-nm luminescence is quenched, but the replacement of the water molecules with thiocyanate or other anions such as acetate, nitrate or halogenides removes the quenching, which makes the complex a responsive probe. The largest effect (a six-fold increase in luminescence) is obtained for thiocyanate, followed by acetate and nitrate (3.5-fold) and chloride (two-fold). 

MnPc-SAMs have been employed for the detection of thiocyanate [86] on SAMs formed by coordination of MPc complexes to preformed SAMs. On MnPc-4-MPy-SAM the oxidation of SCN- occurred at 0.50 V (Table 3). The stability of the electrode was less on MnPc compared to CoPc preformed SAM. Analysis of SCN-in the presence of possible interfering species (uric acid, oxalic acid, and ascorbic acid) in biological samples revealed insignificant effects from these compounds [86], Thus, SCN- can be analyzed in the presence of ascorbic acid. An analysis of the urine of smokers and nonsmokers showed clearly that the SAM electrode could be used to differentiate between the two groups. 

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