Rhenium -, trans

UV irradiation of [cp Re (CO)3] (M = Mn, Re cp = rj -CsHs) in the presence of hydrogen yielded the non-classical dihydrogen complex [cp Re (CO)2 (H2)], but the classical dihydride [cpRe (CO)2 ra 5-H2]. The manganese complex possessed sufficient thermodynamic stability to arrest further oxidative addition at the metal centre. Formation of a rhenium trans-dihydhde complex must have occurred via a c/.s-intermediate. The latter is thermodynamically unstable and the researchers deduced the role of the hydrogen-rich environment in kinetically sustaining the mechanism [12]. 

Figure 24.9 Cluster carbonyls of rhenium containing an encapsulated carbon atom, (a) Octahedral [HjReeC-(C0)i8] . (b) Monocapped octahedral [Re7C(CO)2i] . (c) tran -bicapped octahedral [RegC-(CO)24] . (d) and (e) isomers of [Re7HC(CO)2i] differing in the position of their /r-H atom.

Harrison, W. Trotter, J. (1972) Crystal and molecular structure of tricarbony [(trimethylsilyl)-7r-cyclopentadienyl]rhenium, J. Chem. Soc. Dalton Trans., 678-681. 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

Particularly noteworthy was the rhenium catalysed cross-metathesis of trans-hex-3-ene with vinyl acetate or a,p-unsaturated esters [4]. For example, crossmetathesis of methyl frans-crotonate with frans-hex-3-ene gave the desired cross-coupled product without any self-metathesis of the crotonate (Eq. 2). 

In rhenium compounds that contain two metal-imido ligands, these tend not to occupy positions trans to one another for the same reason. A trigonal-bipyramidal geometry has been identified in 2 and 3, even to the extent that the potentially chelating ligand Ph2PCH2PPh2 acts as a monodentate ligand. 

It would be difficult to envisage entry of Ly into the coordination sphere of rhenium other than at the vacant position trans to the oxo group. It... 

Formulating an acceptable chemical mechanism for each pathway is an interesting challenge. In aid of that, in the reaction with L = Me2SO, intermediate 17 was isolated and fully characterized. It has a structure in which one dmso molecule is coordinated to a rhenium atom of 6 trans to its oxo group. 

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