Complexes of Ammines

Hexammines are more difficult to prepare than the pentammines, one route to Rh(NH3)g+ involving substitution under forcing conditions [86]  

The ethanol is implicated in forming a rhodium(I) complex that catalyses the reaction. The second method produces a mixture 

Kinetic inertness, evidently caused by the electronic configuration, leads to a remarkable unreactivity of the Rh-H bond to hydrolysis. In the absence of air, it is unaffected by ammonia solution in dilute solution, the ammonia tram to hydride is reversibly replaced by water, showing that the hydride has a trans-effect parallel to its trans-influence. 

Both these hydrides insert alkenes and alkynes the crystal structure of [Rh(NH3)5(C2H5)]2+Br2 shows the ethyl group has a trans-influence comparable to that of the hydride [93]. 

The hydroxypentammine is a useful starting material for the nitro and nitrito linkage isomers, the nitrito form separating under mild conditions but transforming to the nitro isomer on standing, especially when heated. 

It is more convenient to start with the triflate ion [Rh(NH3)5(CF3S03)] since triflate is a much better leaving group than chloride and is immediately replaced by liquid ammonia [87]. A third route involves acid hydrolysis of the cyanate complex [Rh(NH3)5(NCO)] , which proceeds quantitatively (probably via a carbamic acid complex). Vibrational studies on Rh(NH3)g assign stretching vibrations as i(Aig) at 514cm , 2(Eg) at 483 cm and z 3(T,J at 472 cm- [88]. 

---

Because of ammine formation, when ammonia solution is added slowly to a metal ion in solution, the hydroxide may first be precipitated and then redissolve when excess ammonia solution is added this is due to the formation of a complex ammine ion, for example with copper(II) and nickel(II) salts in aqueous solution. 

TABLE 11.35 Cumulative Formation Constants of Ammine Complexes at 20°C, Ionic Strength 0.1 ... 

Coordination Compounds. A large number of indium complexes with nitrogen ligands have been isolated, particularly where Ir is in the +3 oxidation state. Examples of ammine complexes include pr(NH3)3] " [24669-15-6], prCl(NH3)] " [29589-09-1], and / j -pr(03SCF3)2(en)2]" [90065-94-4], Compounds of A/-heterocychc ligands include trans- [xCX py)][ [24952-67-8], Pr(bipy)3] " [16788-86-6], and an unusual C-metalated bipyridine complex, Pr(bipy)2(C, N-bipy)] [87137-18-6]. Isolation of this latter complex produced some confusion regarding the chemical and physical properties of Pr(bipy)3]3+ (167). 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

Examples of this coordination number are virtually confined to linear Dock complexes of Cu Ag Au and Hg of which a well-known instance is the ammine formed when ammonia is added to an aqueous solution of Ag+ [H3N-Ag-NH3]+... 

There has been intense study of the complexes of bi- and polydentate ammines since the mid-1970s, driven by interest in the catalytic photodecomposition of water using the excited states of Ru(bipy)g+ (n = 2,3) and related systems (Figure 1.18) [5, 7, 8, 71]. 

Like the ammines, rhodium complexes of ligands like bipy and phen have a significant photochemistry. Therefore, on irradiation, solutions of cw-[Rh(L-L)2X(H20)]2+ (X = halogen) gradually convert to c/s-[Rh(L-L)2X(H20)]2+, though much more slowly than with the ammines [101]. 

Complexes of the bidentate ligand ethylenediamine show similar patterns to simple ammines [151]... 

Platinum ammine complexes have been a fertile area for studying transinfluence. Table 3.21 lists data for a range of ammines showing how /(195Pt-15N) depends upon the trans-atom [153]. (A further selection of data can be found in R.V. Parish, NMR, NQR, EPR and Mossbauer Spectroscopy in Inorganic Chemistry, Ellis-Horwood, Chichester, 1991, pp. 76, 87.) Possibly the most detailed study (of complexes of tribenzylphosphine) examined over a hundred neutral and cationic complexes [154] (Table 3.22). 

The soft Au+ forms relatively few complexes compared with those of phosphines. Complexes with ammines, nitriles and diazoles like Au(NH3)2 and Au(RCN)2 are known but little studied. In linear Au(NH3)2, Au-N is 2.01-2.03 A [70a], [Au(NCPh)2]+ has been used as a labile source of other gold complexes [70b]. AuCl(piperidine) is a monomer with weak tetra-meric association in contrast AuX(py) (X = Cl, Br, I) are [Aupy2]+[AuX2] with a chain structure in the solid state (and Au-Au interactions), suggesting a close balance between factors for molecular and ionic structures [70c] (note also the tetrahydrothiophene complexes in section 4.10.6). 

Fig. 7.63 Mossbauer spectra of some hexacoordinated ammine and pyridine complexes of trivalent iridium taken at 4.2 K with a source of Os in Os metal. The stick spectra indicate the positions and relative intensities of the individual resonance lines (from [285])...

---