Unidentate ammonia

Placing a proton on the X group presumably weakens the Cr-X bond. The enhanced lability is due to a reduced enthalpy of activation, compared with that associated with the step. Normally, the removal of unidentate ammonia or amine ligands from metal complexes is not accelerated by acid, since the nitrogen is coordinately saturated. This situation changes when we consider multidentates (Sec. 4.4.2). 

Figure 1.4 A metal ion surrounded by (a) six unidentate ammonia ligands and (b) three bidentate ethylenediamine ligands. The system with bidentate ligands is more stable, an example of the chelate effect. Triangles represent the ligand interaction sites and the sphere represents a metal ion, such as Ni. ...

The pressure dependence of the reaction of Ni + with NH3 has been studied in greater detail and the pH dependence of the nickel-imidazole reaction has been followed by Letter and Jordan. The complexing of nickel(n) by imidazole is unusual in that the protonated as well as the unprotonated form of the ligand reacts. For most unidentate ammonia- and pyridine-type ligands, protonation removes the only non-bonded electron pair on the ligand and the protonated form is not reactive. However, for imidazole one electron pair is essentially non-bonding even in the protonated form and can be used for complexation (Scheme 1). 

The majority of ligands are either neutral or anionic. Those which coordinate to a metal ion through a single atom are described as monodentate or unidentate. Examples of such ligands which we have encountered thus far include water, ammonia and chloride. A more extensive listing of common ligands is found in Table 1-3. We stress at this point that there is no difference in kind between the interactions of a metal centre with either neutral or anionic ligands. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

The existence of a tetracyanocobaltate(II) ion in aqueous solution has been inferred from studies at high dilution and a brown polymeric complex of composition K2[Co(CN)4] has been isolated from solutions of [Co(SCN)2] and KCN in ammonia.20 Recently, the monomeric air-sensitive complex [(PPh3)2N]2[Co(CN)4] DMF has been crystallized from DMF solution.29 The complex is essentially square planar but does feature a very weak axial interaction to a neighbouring DMF molecule (Co—O separation 264 pm). The [Co(CN)4]2 anion appears to be the sole example of a square-planar low-spin (ji = 2.15 BM) cobalt(II) complex containing solely unidentate ligands. 

It is common practice to consider the traditional Werner octahedral complex ions [MlLNle]" [M = Co(III), Rh(III), Ir(III), Cr(III), Ru(III), Pt(IV) LN = donor atom of unidentate or polydentate ammine or amine] as well as square-planar [M(LN)4p [M = Pt(II), Pd(II)] as kinetically inert compounds. Bound ammonia is generally less labile than bound water, and it has been suggested that this observation can be related to the presence of an extra and exposed electron pair in water. This may make it more sensitive to electrophilic groups in the solvation sheath, which could assist its dissociation from the metal ion (274). If we take the stance of assigning lability as a property of the ligand in such complexes, then ammonia and amines in general can be... 

Unidentate ligands invariably add in a series of steps, as shown here. With multi-dentate ligands, the maximum coordination number of the cation may be satisfied with only one ligand or a few added ligands. For example, Cu(II), with a maximum coordination number of 4, can form complexes with ammonia that have the formulas Cu(NH3)2+, Cu(NH3)i+, Cu(NH3)i+, and Cu(NH3)i+.With the bidentate ligand glycine (gly), the only complexes that form are Cu(gly) and Cufgly) . ... 

Hydrazine ligands are well established and behave similarly to ammonia. In addition to being unidentate they can act as bridges between two metals. There are no authenticated cases of sideways or J -bound N2H4, but two cases of ff -organohydrazine ligands have been reported, e.g. [Mo( =-C5 H5 )(N0)I(NH2 NHPh)] +... 

Two studies have appeared for the base hydrolysis of complex ions of the type ra j-[Co(L)XY] + containing a macrocyclic quadridentate ligand [L = (19) or (20)] and two unidentate ligands (X and Y). When X = NOg and Y = Cl, loss of chloride ion proceeds at much the same rate for L = (19) and (20), the extra methyl groups in (20) having little or no effect upon the base-hydrolysis rates. Complex ions in which X = Y = NH3 have also been prepared and rate data are reported for the base hydrolysis of the first ammonia molecule in 2,6-lutidinium buffers [L = (20), pH 6.0—6.8] or in carbonate buffers [L = (19), pH 9.8—10.6]. This time, the second-order rate constant at 298.2 K increases markedly from 3.2 dm mol s [L = (19)] to 7.6x 10 dm mol s- [L = (20)] as the number of peripheral methyl groups is increased. 

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