The cyanide ligand

For bonding mode MCI the mean CN bond length of 1.140 A is similar to that of organic cyanides (Section III.A.2), thus illustrating the weak Tc-back-donation. The cyanide ligand shows some deviation from linearity, but again is similar to the organic cyanides. 

TABLE 21. Mean geometric parameters for the cyanide substructures MCI and MC2 

MNC angle covers a much wider range from 155 to 180°. For all fragments the M—C bond length is shorter than the M—N bond. In some cases this difference is small, but for 7 fragments the difference is greater than 0.35 A, thus reflecting the polymeric nature of these structures. 

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A series of complexes in which the cyanide ligands are modified or replaced arises from the decomposition of methyl and pyridiomethylcobalt(111) pentacyanide derivatives in acid solution. The reactions include protonation of a cyanide ligand, insertion of a cyanide ligand between the organic group and the cobalt atom to produce an imine (see Section VI,D), decomposition of this imine to an acyl product, and replacement of a cyanide ligand by water 100, 101). The products are listed in Table III, 29. 

A similar effect conceivably accounts for the higher Pi value (—0.74 V) (weaker net electron-donation) of the cyanide ligand estimated [15] at trans- FeH(dppe)2 with the strong donor trans-hydride, in comparison with that (—l.OV) [10] obtained at Cr(CO)5 and also proposed at trans- MoL(dppe)2) (L = CO, N2) with the strong net electron-acceptor L ligand, in spite of the lower electron-richness of the former Fe site Eg = 1.04 V) relatively to the latter (Eg = —0.11 or —0.13 V) molybdenum centers. 

X-Ray diffraction shows that the [Pt(CN)6]2 ion is regular octahedral with linear Pt—C—N bonds.3"2 The IR and Raman spectra of K2[Pt(CN)6] have C—N stretching vibrations at higher frequencies and C—N force constants which are larger than the corresponding divalent complexes.303,304 This correlates with less n back-donation from the metal to the empty n orbitals of the cyanide ligand in the PtIV complexes. 

Alkylation of the cyanide ligands in Ag4M(CN)8 with RX (X = halides) has provided the only route to the complexes M(CN)4(CNR)4 (M = Mo, W R = alkyl, allyl, CH2Ph, CPh3) (72, 73), as direct reaction of isocyanides on the cyanide anions has given only reduced products (74). 

The chemistry of osmium is dominated here by the cyanide ligand there is one fulminato complex, and a number of species containing Os—Sn bonds. 

The cyanide ligands in [Fe(CN)2(LL)2] (LL = phen, bipy) are sensitive towards protonation successive protonation of both cyanide ligands causes the colour of the complex to change from violet through orange-red to yellow.508 509 Protonation in liquid hydrogen fluoride has also been examined in detail,510 and a cis configuration confirmed for the diprotonated cation. Both... 

In certain cases, a bound cyanide ligand can participate in an oxidation-reduction reaction. For example, the electroreduction of the complex mans -[Mo(CN)Cl(dppe)2] in the presence of phenol reduces the cyanide ligand to the simplest aminocarbyne ligand, =CNH2, and gives trans-[(Mo=CNFl2)Cl(dppe)2]. The reduction of the cyanide is reversible, as oxidation of the aminocarbyne complex regenerates trans- [Mo(CN)Cl(dppe)2]. 

The cyanide ligand is often considered as a pseudohalide (see Pseudohalide). Fusion of (NH4)3 [IrCle] with KCN gives K3[Ir(CN)6], which can be isolated as pale yellow crystals after recrystallization from water. Derivative anions of the type [Ir(CN)5X] (X = halide or hydroxide, n = 3 X = MeCN, n = 2) can be prepared as shown in Scheme 7. 

The cationic dinitrosyl complexes (146) are apparently much more electrophilic than (143). Studies of the reactivity of (146) have been focused on attack of coordinated organic ligands of other transition metal complexes, such as the cyclooctatetraene ligand in L3M(cot) complexes (M = Fe, Ru) and Cp Co(cot) complexes (Cp = Cp, Cp ), the cyclopenta-dienyl ligand of CpRh(cot) complexes, and the cyanide ligand... 

Resonance Raman (Xexcit = 647 nm) and mid-infrared spectra of the cyanide-bridged model compounds display three isotope-dependent vibrational modes. Two vibrations (2182 and 535 cm ) show monotonic decreases with increasing mass of the cyanide ligand and could thus be assigned to the C-N stretch and Fe-CN-Cu stretch, respectively. The C-N stretch was also observed by resonance Raman for the first time. The third vibration, detected with resonance Raman, showed a zigzag-type behavior (495,487,493,485 cm ) with the isotopomers that... 

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