TT-Bonded Ligands

In contrast to the metal-ligand bonds shown in Section 1.1, which are much like main group Lewis acid-base interactions, many of the ligands in organometallic chemistry 

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Silver ions form a number of complexes with both TT-bonding and non-TT-bonding ligands. Linear polynuclear complexes are known. The usual species are AgL and AgL2, but silver complexes up to AgL have been identified. Many of these complexes have commercial appHcation. 

In the second case, the phosphorus atom in the phosphine also has empty d orbitals that can accept electron density donated from the Pt2+. In fact, it is more effective in this regard than is the sulfur atom in SCN-. This results in the more stable bonding to SCN- being to the nitrogen atom when PR3 is in the trans position. In essence, the presence of tt bonding ligands in trans positions that compete for back donation leads to a complex of lower stability. As will be discussed in Chapter 20, this phenomenon (known as the tram effect) has a profound effect on the rates of substitution reactions in such complexes. 

Other P=E transition metal Tt-bonded ligands are not as numerous, but several X-ray crystal structures are available. The three-coordinate Pv ligand R2N(RN)PS forms the planar platinum(O) complex, (57), where the P=S bond shows bond lengthening on coordination.304 (The selenium analogue is shown in Section 14.5.3.2.)... 

Organochromium species containing tt-bonded ligands were first prepared by Hein as early as 1919, but were not definitively characterized for another 35 years. Hein reported on the reaction of PhMgBr with CrCb to give Chromorganoverbindungen this synthesis was reproduced by Zeiss and Tsutsui in 1954, at which time they identified the products as bis(arene)chromium cations. Shortly thereafter, Fischer and Haftier published their route to neutral bis(benzene)chromium. This early work has been chronicled, most recently by Seyferth. ... 

TT-Bonded ligands can be closed shell, 2n electron donors (n = 1 — 3) such as alkenes, allenes, alkynes, arenes, and polyenes these have been termed even polyene ligands. They may also be open shell, odd polyene ligands such as allyl, pentadienyl, cyclopropenyl, and cyclopentadienyl, which are formally viewed as anionic hgands, donating 2n electrons (n = 1 - 3). 

ORGANIC SYNTHESIS USING TRANSITION METAL COMPLEXES CONTAINING tt-BONDED LIGANDS... 

However, over the last 60 years a new type of chemistry has emerged. Although the first examples, tertiary arsine complexes, were initially prepared as an extension of classical rhodium(III) chemistry, the newer complexes containing tt-bonding ligands have been a consequence of the intense interest in the catalytic properties of rhodium(I) complexes. Examples of these ligands also include tertiary phosphines and stibines, although it is debatable to what extent they act as r-acids when coordinated to rhodium(III). 

Asymmetric Synthesis by Homogeneous Catalysis Carbonyl Complexes of the Transition Metals Hydride Complexes of the Transition Metals Organic Synthesis using Transition Metal Complexes Containing tt-Bonded Ligands Polynuclear Organometallic Cluster Complexes. 

In a first discussion of the possible mechanism (Schrauzer, e< al., 32, 33) it was assumed that the products are formed within tt complexes in a concerted fashion ( it complex multicenter reaction ). Norbom-adiene forms many complexes with transition metals in which it is symmetrically coordinated. Some pertinent examples are (XII) (1, 2), (XIII) (35), and (XIV) (36). Ni(CO)4 reacts with acrylonitrile to produce bisacrylonitrile nickel, Ni(CH2=CHCN)2 which adds phosphines forming 1 1 and 1 2 adducts (37, 38). Acrylonitrile is a relatively strong TT-bonding ligand and may back-accept charge from the metal via d -p bonding. In this regard it is related to duroquinone (tetra-methylquinone), which is known to form Ni(0) it complexes of the type Ni(Dqu)2 or (XIV). The most likely intermediate in the reaction... 

Formation and coordination of H5C5 as a cr-bonded ligand and as a TT-bonded ligand. In the latter mode, the ligand occupies the face of an octahedron in the same way that a conventional cyclic triamine does. 

As in (r -C5H5)2Cr, the benzene ligand in (r -C5H5)Cr(CO)3 can be lithiated (equation 23.108) and then derivatized (scheme 23.109). The reactivity of halfsandwich complexes is not confined to sites within the tt-bonded ligand equation 23.110 illustrates substitution of a CO ligand for PPh3. 

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