Reactions of H2S with metal complexes

In a different approach to this problem, several groups have studied the reactions of HjS with a variety of metal complexes. Only a limited number of complexes containing intact coordinated HjS have been prepared, as can be seen in the data contained in Table 

Some of them have been shown to exist only in solution by in situ spectroscopic measurements or by indirect methods, and very few have been structurally characterized [3, 22-37]. 

P-N = PhjPCjHjNMejji ThiCp = C,H,S-CH,-C,H, ctipdp = N-ethyl(ietraisoppropoxy)diphosphazane X-ray structure available. 

The main reason for the scarcity of HjS complexes seems to be mainly the ease with which they can be oxidized, and indeed, their stabilization is usually accomplished by use of electron-rich metal centers in combination with bulky ligands that provide a rigid protective environment. Still, the binding of H2S to metal ions is in general rather weak, and its displacement by other Lewis bases is very favored. Nevertheless, once it is coordinated H2S is, as expected, activated toward a number of reactions. For instance, deprotonation can take place by the action of strong bases (Eq. 5.12), and several examples of the oxidative addition of HjS to give hydrido-hydrosulfido species are known some of them probably proceed through undetected M-SHj intermediates, as indicated in Eq. 5.13. 

In some cases the oxidative addition reaction has been observed to proceed further with the elimination of dihydrogen and the consequent formation of a stable sulfido complex, in a way the reverse reaction of the activation of hydrogen by Cp 2Ti=S described in the preceding section. For instance, the Zr(n) complex Cp 2Zr(CO)2 reacts with excess HjS in the presence of pyridine to yield the corresponding bis(hydrosulfido) derivative Cp Zr(SH),. This reaction takes place via an initial oxidative addition of HjS to yield Cp Zr(H)(SH) followed by hydrogen elimination in the presence of pyridine to form a terminal sulfido complex Cp Zr(S)(py) the latter undergoes a second oxidative addition of H2S to finally produce the bis(hydrosulfido) complex, as shown in Eq. 5.14 [2]. 

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The bonding of H2 in metal complexes was described in Chapter 16. In connection with the oxad reaction in which the bonding is not static, it can be presumed that the o orbital on the hydrogen molecule functions as an electron pair donor to an orbital on the metal atom. Simultaneously, the o orbital on the H2 molecule receives electron density from the populated d orbitals on the metal atom as a result of back donation. The result is that two M-H bonds form as the H-H bond is broken in a process that is accompanied by a very low activation energy. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

Unlike H2O, H2S rarely forms simple complexes with transition metal ions owing to facile deprotonation to HS and. Additionally, reaction of H2S with simple metal salts... 

The reactions of late transition metal complexes with H2 are usually explained by oxidative addition of H2 giving dihydride. However, in certain reactions of transition metal alkyls or acyls with H2 or boranes, involvement of <7-bond metathesis better accounts for the results. 

H in combination with other ligands. Such complexes may be made in a variety of ways. Probably the most common synthesis is by reaction of a transition metal complex with H2. For example,... 

Given the importance of catalytic hydrogenation reactions, it is not surprising that the oxidative addition of H2 to metal centers is among the best-studied systems. The prototypical reaction of H2 with Vaska s complex to yield the Ir(lll) dihydride derivative Ir(PPh3)2H2(CO)Cl (Scheme 6) displays second-order kinetics, a negative entropy of activation (i.e., an observation consistent with the intermediacy of a a-complex), overall CM-stereochemistry, and negligible solvent effects. 

In Scheme 1 is represented an idealized picture of the two possibilities for the hydrogenation of alkenes by metal complexes not containing an M—11 bond. One possibility involves initial coordination of the alkene followed by activation of H2 (alkene route). The other (more general) possibility is the hydride route, which involves initial reaction with H2 followed by coordination of the alkene. The second general mechanism, usually adopted by catalysts containing an M—H bond, is shown in Scheme 2. 

The preparation and reactions of metal cluster ions containing three or more different elements is an area with a paucity of results. The metal cyanides of Zn, Cd (258), Cu, and Ag (259) have been subjected to a LA-FT-ICR study and the Cu and Ag complex ions reacted with various reagents (2,256). The [M (CN) ]+ and [M (CN) +11 ions of copper, where n = 1-5, were calculated to be linear using the density functional method. The silver ions were assumed to have similar structures. The anions [M (CN) +1 of both copper and silver were unreactive to a variety of donor molecules but the cations M (CN) H + reacted with various donor molecules. In each case, where reactions took place, the maximum number of ligands added to the cation was three and this only occurred for the reactions of ammonia with [Cu2(CN)]+, [Cu3(CN)2]+, [Ag3(CN)2]+, and [ Ag4(CN)3]+. Most of the ions reacted sequentially with two molecules of the donor with the order of reactivity being Cu > Ag and NH3 > H2S > CO. 

Supercritical fluids allow the formation of species that cannot be made in conventional solvents. For example, rj2-H2 complexes have been generated by direct reaction of hydrogen with a transition metal carbonyl complex [10]. In order to isolate these compounds, a continuous flow reactor was used and such compounds could be isolated with surprising ease. 

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