Thiocyanates five-coordinate

The azide and thiocyanate complexes [CoLX], where L is a dinegative N2S3 donor ligand making the five-coordinate [CoL]+ moiety resemble... 

Rate constants for the replacement of water by azide or thiocyanate from the five coordinate (2tyr 2his, 1H20) copper center in Fusarium galactose oxidase decrease with increasing pH, due to the greater difficulty of displacing OH- (312). 

The second critical test of this conjugate base mechanism is based on the fact that this five-coordinated intermediate, if indeed it exists, would not always have to react with the solvent, though the solvent would be what it would react with under most circumstances. We have run this type of base hydrolysis in the presence of many anions of high concentration, and the only thing that we can find is the hydroxo complex so at least in water solution, water seems to be what this five-coordinated intermediate picks up. But in dimethylsulfoxide it certainly is possible to throw in various anions, and since dimethylsulfoxide is not as good as water in coordination, other nucleophiles may react. We do find in dimethylsulfoxide that a base, such as hydroxide ion, speeds up the rate of base hydrolysis but the product, instead of being a hydroxo compound, is the complex corresponding to whatever anion we have added, such as nitrite ion, azide ion, and thiocyanate ion. 

In the acid hydrolysis of the pentamminecobalt complexes where you have a leaving group, such as nitrato or bromo, H. Taube and A. Haim came out with some very interesting work, with which I am sure you are all familiar. They suggested that the five-coordinated pentamminecobalt species was formed which then discriminated between various nucleophilic reagents, sometimes reacting with water, sometimes with- thiocyanate ion. In fact, they were able to measure these nucleophilic discrimination factors in a number of cases, and they were able to correlate different types of reactions in which the pentamminecobalt would be generated in different ways. 

A more satisfactory solution to the mechanism of these substitutions now seems experimentally feasible. It is likely that the trisamino chelate (XXXIII) could be completely resolved by salt formation with a suitable optically active acid. The optically pure amine could then be converted by electrophilic cleavage into optically active bromo-, chloro-, and thiocyanate-substituted chelates. It would thus be a simple matter to determine whether these substitutions proceed with complete retention of asymmetry. Further, the question of a symmetrical five-coordinate intermediate in racemization of such compounds could probably be elucidated by a study of solvent polarity or salt effects on the kinetics of the racemization of these chelates. 

The reaction of a 1,10-phenanthroline complex of iridium, [Ir(cod)-(phen)]+, with dioxygen in methanol solution has been studied (38). When the anion for this cationic complex is chloride, no anion-cation interaction occurs, and the iridium system remains four-coordinate. However, when either iodide or thiocyanate is present due to the addition of their sodium salts (or in the presence of added triphenylphos-phine when the anion is chloride), the iridium system becomes five-coordinate because of the interaction between I", SCN", or PPh3 and the iridium center. These five-coordinate systems react more rapidly with dioxygen than did the four-coordinate system at both normal and elevated pressures. An end-on oxidative addition of the dioxygen moiety, with displacement of the , SCN, or PPh3 ligands, was postulated. 

Bis(2-dimethylaminoethyl)methylamine]Cd(NCS)2 contains five-coordinate cadmium in a square pyramidal environment the apical position is occupied by N-bonded thiocyanate (Cd—N = 2.18 A) while the basal positions are occupied bv the amine (Cd—N = 2.34-2.37 A) and the second thiocyanate (Cd—N = 2.21 A) ligands.217... 

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. 

The thiocyanate complexes [Pd(CNS)2(L—L)2] are known for some bidentate ligands L—L in general the tendency to five-coordination is markedly less than for the corresponding halide complexes, although the same order with respect to donor is followed.49,50... 

Burmeister and colleagues have described the related pseudohalogen derivatives MfterpyjXj (X = SCN or SeCN) (90-92). The platinum compound exhibits the two thiocyanate stretching frequencies expected for a square-planar complex, and is formulated [Pt(terpy)(NCS)][NCS], However, the palladium complexes are less easily formulated, exhibiting absorptions due to coordinated ECN (E = S or Se) only. These observations were interpreted in terms of a square-planar structure, with a bidentate terpy ligand in view of the known ability for palladium and platinum diimine complexes to form five-coordinate species, this formulation must also be considered. In the absence of definitive structural evidence, the formulation as five-coordinate species must be regarded as speculative. 

Many simple complexes have been prepared as models of active sites of biomolecules. For example, a reactive five-coordinate thiolate Co complex (Figure 24) was prepared to model the active site of nitrile hydratase, a Co or Fe metalloenzyme that promotes the conversion of nitriles to amides. The synthesized model complex is facile in its uptake and release of azide and thiocyanate, indicating that an appropriate nonleaving group environment enhances ligand displacement sufficiently for catalytic paths in non-redox active Co metalloenzymes. Other examples have appeared earlier in this report. 

The five-coordinate zirconocene dichloride 1178 contains a tethered bis(Cp)-phosphine ligand it was prepared by salt metathesis of ZrCl4(THF)2 with Li2[PhP(CFI2CF[2C5F[4)2].911 Coordination of the phosphorus atom to the metal center was confirmed by its X-ray structure. Dissolution of 1178 in wet MeOH followed by evaporation of the solvent yielded the cationic chloro-aqua complex 1179 shown in Scheme 277 as the chloride salt. Treatment of an aqueous solution of 1178 with excess potassium thiocyanate gives the bis(isothiocyanato) complex 1180. 

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