Multiply bonded complexes

Mixed alkyl-carboxylato complexes of dirhenium(III) have been prepared starting from either Re2(02CMe)4Cl2 or [Re2Me8]2. The chemistry of these multiply bonded complexes (e.g. Re2(02C-Me)2R,i and Re2(02CMe)4Me2) is discussed elsewhere (see refs. 5 and 6). 

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 Tris(pyrazolyl)borates) is a Cp analog." It reacts with Mo(CO)6 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 stracture. These reactions are summarized in equation (16). 

Some of the most interesting metal-metal bonded complexes contain multiple bonds. A good example" " of this, and one of the first multiply bonded complexes formed, is the reaction of pentamethylcyclopentadiene with molybdenum carbonyl (equation 19). 

A range of complicated reactions can occur between metal-metal multiply bonded complexes and ligands that... 

The chemistry of metal-to-carbon multiply bonded complexes continues to flourish. The critical role olefin metathesis chemistry now plays in so many areas of synthesis and materials science has provided important impetus for the continued discovery and study of such species. " While the scope of this contribution is far too narrow to even begin to touch upon this broad area of organometallic chemistry, the authors wish to highlight one new class of organometallic complexes that falls within this category, that of the terminally bonded carbides. 

The octachlorodirhenate(III) anion is the key starting material for entry into the chemistry of multiply bonded complexes of dirhenium. To date, the... 

In a recent review on predetermined chirality at metal centers, Khof and von Zelewsky described the chiral quadruply bonded species A and A-[Mo2Cl4(5,5-dppb)2] (where dppb = 2,3-bis(diphenylphosphino)butane) as M-configurational double helices. Such twisted multiply bonded complexes are paradigm examples of intrinsically chiral chromophores where the chiral center is not located at the metal atom but embraces the whole twisted M02X4P4 unit. The CD spectra of a range of chiral quadruply bonded compounds, with a variety of bidentate and monodentate chiral ligands (phosphines and amines) have been reported and can be explained by a simple metal localized theory. ... 

The compound K2[Re2Cl8] is special amongst the now thousands of examples of compounds with metal-metal quadruple bonds, as the discovery of its short Re-Re bond (2.24 A) in its solid-state structure reported 50 years ago led to the recognition of four-fold bonding in these types of compounds. Dirhenium compounds have been intensively studied in the subsequent decades since this report, and have played an important role in improving our understanding of electronic structure and bonding in metal-metal multiply bonded complexes. ... 

Stereochemical aspects of reactions of complexes of d° transition metals with multiply bonded ligands. Y. V. Kokunov and Y. A. Buslaev, Coord. Chem. Rev., 1982, 47,15-40 (120). 

Phosphorus, arsenic, antimony and bismuth multiply bonded systems with low coordination number — their role as complex ligands. O. J. Scherer, Angew. Chem., Int. Ed. Engl., 1984, 24, 924 (85). 

Consistent with the involvement of the boron 2pz-orbital in this transition is the almost total reduction of the long-wavelength absorption of (PhCH2)3B above 250 nm by its complexation with ammonia. Strong chemical support for a multiply bonded boron in the excited state (48b) can be deduced from photolysis results with tribenzylborane (49).48 While photolysis of... 

As in main-group chemistry, hypovalent hydrides of the transition series have pronounced tendencies to form bridging tau bonds. In addition to H4TaTaH4, the multiply bonded species HTaTaH features two symmetrically bridging hydrides. Despite the complexity introduced by such tau bonds, the Ta-Ta interaction clearly has high bond order, as simple Lewis-like structures prescribe. 

Electrochemical data for a number of multiply bonded Rc2 complexes are represented in Table 2 [42-50]. The oxidation of quadruply bonded Re2 pro-... 

---