Triply bonded structural complexity

In addition to substitution of the carbonyl groups, changes in the cyclopentadienyl ring have also been extensively studied. The use of pentamethylcyclopentadiene led to formation of some of the first organometallic multiply bonded complexes as discussed in Section 4.8. The tris(pyrazolyl)borate ion (Tp, see Ttis(pyrazoIyI)borate is a Cp analog. It reacts with Mo(CO)e to yield the TpMo(CO)3 anion. Oxidation yields the paramagnetic radical that shows no tendency to form a single metal-metal bond. Decarbonylation yields a triply bonded structure. These reactions are summarized in equation (16). 

There is currently a paucity of structural data for binuclear iron complexes in other oxidation states. Wieghardt et al. have reported the structure of the only triply bridged diferrous complex thus far, [(Me3tacnFe)20H(OAc)2] (7,8). Its structure is closely related to that of the corresponding difertic complex, with Fe-p.-OH bond lengths of 1.99 A and an Fe Fe separation of 3.4 A. The two metal centers are antiferrcMnagnetically coupled with a J value of -13 cm . ... 

T.M. Gilbert et al., Synthesis and electronic properties of triply bonded hexakis(fluoroalkoxy) dimolybdenum complexes. Structure of Mo2[OCMe(CF3)2]6 and investigation of the nature of the frontier orbitals in triply bonded M2X6 compounds. Inorg. Chem. 31, 3438-3444 (1992)... 

The THT and SMe2 adducts have structures of the type (18-B-V). Their chemistry has been extensively studied and it is summarized in Fig. 18-B-7. The diverse, and in some cases unique, reactivity of these compounds includes substitution with preservation of the geometry or with conversion to (MX4)2(/t-X)2 species, oxidative-addition,53 cluster formation, splitting of C—N bonds,54 and above all coupling of the molecules with triply bonded carbon atom.55 They catalytically trimerize and polymerize terminal acetylenes, and dimerize nitriles and isonitriles with incorporation of the new ligand into the complex. Another remarkable reaction of M2C16L3 is the metathesis of M=M and N=N bonds into two M=N bonds upon reaction with azobenzene. 

Figure 14 Crystal structure of the triply bonded V2(0, 0) complex [(776-C6H5CH3)V(CO)2]2 (reproduced by permission of the American Chemical Society from Organometallics 1993, 12, 1682-168 )).

A recent exhaustive review [1] presents the known and well characterised lanthanide hydrides. Recently, a few additional original complexes have been synthesised. These include phospholyl [7] and indenyl derivatives [8], a bimetallic metallocenic structure (Scheme 1) [4], a samarium hydride supported by a calix-tetrapyrrolyl ring, with a triply bonded hydride ligand (Scheme 2) [9], metallocenes with chiral groups which has been used for the asymmetric hydrogenation [10], a tris(pyrazolyl)borate ytterbium(II) hydride [11], Cp amido-[12] and bis(amido)yttrium hydrides [13]. The latter was not isolated but obtained in situ from the methyl derivative (Scheme 2). 

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