Cyanide complexes, linkage

Many studies have been carried out on this reaction. Some of the unusual features of this reaction will be described in Chapter 20. Note that in writing the formulas for linkage isomers, it is customary to write the ligand with the atom that functions as the electron pair donor closest to the metal ion. Special consideration will be given in Chapter 20 to the behavior of cyanide complexes because CN-is also an ambidentate ligand. 

The exceptional strength of the metal-cyanide linkage makes it challenging to develop catalytic processes that involve excess CN" as reagent. The Pd-catalyzed cyanation of aryl halides discussed in Chapter 19 to form aryl nitriles initially suffered from the formation of catalytically inactive cyanide complexes containing multiple CN" ligands the reaction has been improved by the use of ZnfCN) or Kj[Fe(CN)J as the CN" source. ... 

Nitroprusside forms both dinuclear and trinuclear complexes with aquacobalamin (Bj2a) [131]. Coordination involves a Fe-C=N-Co link, with linkage involving trans cyanides in the trinuclear complex. Such complexes are slower acting as hypotensive agents than nitroprusside itself [7]. 

The [Ni(CN)4]2 anion is one of the most stable nickel(II) complexes and an overall formation constant as high as about 1030 has been determined.627,62 The structure of the complex is square planar with the nickel(II) bound to carbon atoms of cyanides and with linear Ni—C—N linkages (Table 37).629 630 The planar [Ni(CN)4]2 units are stacked in columns in the crystal lattice with Ni—Ni interlayer distances as short as 330 pm. C-bonded CN- is a strong field donor and the electronic spectrum of [Ni(CN)4]2 shows two weak d-d bands at 444 and 328 nm. 

COORDINATION COMPOUNDS. One of a number of types of complex compounds, usually derived by addition from simpler inorganic substances. Coordination compounds are essentially compounds to which atoms or groups have been added beyond the number possible on the basis of electrovalent linkages, or the usual covalent linkages, to which each of the two atoms linked donates one electron (o Form the duplet. The coordinate groups are linked to the atoms of the compound usually by coordinate valences, in which both the electrons in the bond are furnished by the linked atom of the coordinated group. The amines and complex cyanides are representative of coordination compounds. 

While bifunctionality is known for the halogens and many pseudohalogens, it is most pronounced for cyanide and influences the structures, properties and chemistry of many of its derivatives. Cyanide bridges were present in the first recorded synthetic inorganic complex, Prussian blue (ca. 1700), and cyanide linkage isomers were often proposed in the old literature but reasonable evidence for the existence of linkage isomers and the structure of Prussian blue is very recent. 

B. Complexes Exhibiting Cyanide Linkage Isomerism / 253 IV. MAGNETIC PROPERTIES... 

A clear case of cyanide linkage isomerism was established for the pentanuclear TBP complex [Fe tmphen)2]3[Cr (CN)g]2 (174a). Unlike the similar complexes [Fe°(tmphen)2l3 [M ° CN)g]2 (M = Co or Fe), which exhibit temperature induced transitions between the high- and low-spin state at the equatorial Fe(II)... 

There are Cr° and Cr complexes of both aryl and alkyl isocyanides (RNC) as well as cyanides because of their Cr-C linkages, these are described elsewhere see Chromium Organometallic Chemistry). 

Linkage isomerism is but a special case of ambidentate behavior in ligands. The cyanide ion provides good examples of such behavior. In discrete complexes it almost always bonds through the carbon atom because of the. stronger tt bonding in that mode. It has also been reported to form a few linkage isomers such as cjj-[Co(lrien)(CNK] and c/4 4Co(trien)(NC)j]. ... 

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