NCS ligands

The trarax-dichloro and dithiocyanate complexes show MLCT transitions in the entire visible and near IR region. The lowest energy MLCT transition band of the trara-dichloro complexes is around 700 nm in DMF solution, and the complexes show weak and broad emission signals above 950 nm. The absorption and emission maxima of the Zrarax-dithiocyanate complexes are blue shifted compared to its trarax-dichloro analogues due to the strong -k acceptor property of the NCS- ligands compared to Cl-, which is consistent with the electrochemical properties of these complexes. 

Therefore, sulfur coordination of NCS ligand to ruthenium shows a carbon resonance at around 120-125 ppm, compared to the nitrogen coordination of NCS ligand, where the 13C resonance is in the region of 130-145 ppm.46... 

As a further illustration of the dependence of n i 7t pi-backbonding interactions on metal and ligand character, we may compare simple NiL complexes of nickel with carbonyl (CO), cyanide (CN-), and isocyanide (NC-) ligands, as shown in Fig. 4.41. This figure shows that the nNi 7rL pi-backbonding interaction decreases appreciably (from 28.5 kcal mol-1 in NiCO to 6.3 kcalmol-1 in NiNC-, estimated by second-order perturbation theory) as the polarity of the 7Tl acceptor shifts unfavorably away from the metal donor orbital. The interaction in NiCO is stronger than that in NiCN- partially due to the shorter Ni—C distance in the... 

Diphenyl sulfoxide complexes of uranium(VI) have been reported (495) and a thorough infrared and conductivity study detailed (433), suggesting that the thiocyanate complexes of empirical formula [U02(R2S0)2(NCS)2] are in fact dimeric with bridging iV S-NCS ligands. 

Bis(t-butylisocyanide)tetraphenylchlorinatoiron(III) (chlorin = dihydroporphyrin) is a low-spin iron(III) complex with the unusual ground state (d z, dy dxy). This deviation from normal behavior, viz (d z, dy dxyf, is ascribed to the strong rr-acceptor properties of the axial Bu NC ligands." ... 

The photochemical and thermal stabilities of Ru complexes have been investigated in detail [8,153-156]. For example, it has been reported that the NCS ligand of the N3 dye, cri-Ru(II)(dcbpy)2(NCS)2 (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), is oxidized to produce a cyano group (—CN) under irradiation in methanol solution. It was measured by both ultraviolet-visible (UV-vis) absorption spectroscopy and nuclear magnetic resonance (NMR) [8,153]. In addition, the intensity of the infrared (IR) absorption peak attributed to the NCS ligand starts to decrease at 135°C, and decarboxylation of N3 dyes occurs at temperatures above 180°C [155]. Desorption of the dye from the 2 surface has been observed at temperatures above 200°C. 

It has been considered that the high stability of the dye in a DSSC system could be obtained by the presence of I - ions as the electron donor to dye cauons. Degradation of the NCS ligand to the CN ligand by a intramolecular electron-transfer reaction, which reduces consequently the Ru(III) state to the Ru(II) state, occurs within 0.1-1 sec [153], whereas the rate for the reduction of Ru(in) to Ru(II) by the direct electron transfer from I ions into the dye cations is on the order of nanoseconds [30]. This indicates that one molecule of N3 dye can contribute to the photon-to-current conversion process with a turnover number of at least 107—10s without any degradation [153]. Taking this into consideration, N3 dye is considered to be sufficiently stable in the redox electrolyte under irradiation. 

The complex [ZnL2(NCS)2] (L = benzoquinoline) has been prepared and the NCS ligands have been shown to be N bonded, in contrast to those in the corresponding Hg11 or Cdn complexes.487... 

An example of the few l,l-/ -S-bonded thiocyanate complexes is the recently reported Os3 Au(SCN)(CO) o(PPh3) (96).229 The S atom bridges two Os atoms while the almost linear NCS ligand makes a considerable angle with the Os2S plane (cf. canonical structure 85). 

Thiocyanate complexes of nickel are likewise found to contain the NCS ligand either terminal N-297,298 or 1,3- -bonded.299 The NCS fragment in the polymeric structure of Ni(NCS)2232 is 1,1-p-S bridge bonded over two Ni atoms and N-bonded to a third Ni atom. 

Nice examples of the ambident character of the NCS ligand are provided by the various structures reported for Pd and Pt thiocyanate complexes. Structures are known containing terminal N-bonded NCS,225,294,300 terminal S-bonded225 292-294 300 301 as well as 1,3-fi bridging NCS.293,302... 

Thiocyanate complexes of copper are a further illustration of the versatile bonding behavior of the NCS ligand terminal N e.g. 107),307-312-322 terminal S (108),308 bridging 1,3-ft (e.g. 109)309 310-323 and 1,1-ju-S e.g. 110)311,312 have been reported. In the recently reported structure of a copper NCS complex both terminal and 1,1-fi-bonded NCS fragments are present with as a special structural feature a considerable bending (140°) in the NCS ligands.312... 

The replacement of one NCS ligand in [Fe(bipy)2(NCS)2] by a py ligand, which is known to possess stronger 7r-back-bonding properties than NCS, will cause an increase in the ligand field strength. 

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