L-type ligands

A note on symbolism used throughout the remainder of the text is in order. Recall from Chapter 3 that the symbol L represents a ligand that donates two electrons to a complex (e.g., CO or phosphine). X (or sometimes Y or Z) represents a one-electron donor such as Cl or alkyl. To indicate a collection of unspecified L-type ligands (or sometimes L and X ligands) attached to a metal, the symbol L is used. 

The rate of substitution of a particular ligand is a function of ligand type. Carbon-donor L-type ligands, such as CO or arene, dissociate rather easily because they are neutral in the free state. L X groups (such as Cp), on the other hand, dissociate more reluctantly as radicals or ions that are less stable. 

Coordinatively-saturated metal-alkyl complexes tend not to undergo (3-elimination, especially if they are in solution in the presence of an excess of L-type ligand, such as CO or PR3, or if ligands also bound to the complex, such as Cp or bidentate phosphines, are reluctant to leave and provide an open coordination site. 

It is this strong nucleophilicity that makes NHCs effective L-type ligands, and their electronic and steric properties rival those of phosphines. Figure 10-6 displays several of the most common NHCs, which are used as ligands in transition metal complexes. 

Although transition metal complexes do not usually react directly with free carbenes (with the exception of the formation of NHC-metal complexes, Section 10-2-3), low-valent Group 7 to 9 metal complexes in particular react with diazoalkanes to produce alkylidenes. Equation 10.17 shows a general example of this reaction. The complex must either be unsaturated or possess a labile L-type ligand so that the reaction can occur. The intermediate in this reaction is unlikely to be a free carbene. 

Observation 1 indicates that the overall charge is important in determining the reactivity of Group 7-9 carbene complexes adding a positive charge makes the Os complex electrophilic. The Ru complexes compared in observation 2 differ in oxidation state of Ru (assuming that the carbene is an L-type ligand), with the latter an electrophilic Ru(II) complex and the former nucleophilic and Ru(0). The Re complex, described in observation 3, is transitional between nucleophilic and electrophilic reactivity. There apparently is a balance... 

Path a shows loss of an L-type ligand first (giving complex 56), which allows complexation of the alkene to the metal to yield 57. Rearrangement of 57 to metal-locyclobutane 58 amounts to a formal 2+2 cycloaddition of alkene to compound 56. Intermediate 58 can then undergo RE to give cyclopropane 59, or it may decompose to give 60 and a new alkene 61. Cyclopropanation is stereospecific with respect to the substitution pattern of the alkene, but two stereomeric products (59a and 59b) are possible if two different substituents were originally attached to Cc.irbcnc. 

L M A generalized metal complex fragment with n L-type ligands attached... 

Examples of stable q (C=C) complexes which incorporate thiophenes as olefin-like 2-electron donor (L-type) ligands are very scarce, having been characterized only in a few cases. Taube reported the magnesium reduction of [OsfNHjljfOTf)] in the presence of thiophene to yield Os(NH3)5[q C=C)-T] 2 , which was characterized by NMR spectroscopy [47]. Several analogues with a variety of thiophenes, viz. [Os(NH3)5[q (C=C)-Th]) (Th = 2-MeT, 3-MeT, 2,5-Me2T, 2-MeOT, and BT), were isolated in a pure form by Spera and Harman from similar reactions and characterized by NMR spectroscopy [48], These complexes readily add electrophiles to the sulfur atom and the resulting adducts react further with a variety of nucleophiles (H, CN , OAc , py,... 

Due in large part to the development of ruthenium catalysts, olefin metathesis reactions can now be carried out on a diverse array of functionalized electron-rich and electron-poor olefins. As we have described, mechanistic analysis was instrumental in the design of more highly active second generation catalysts with expanded substrate scope, which was achieved by proper differentiation of the two L-type ligands within the (L)2(X)2Ru=CHR framework. Further investigations have revealed that these new catalysts display several unexpected features, and mechanistic analysis continues to be an invaluable tool for understanding reactivity patterns and for the development of new catalyst systems. 

It should be noted that in some of the examples given above, the radical centre also possesses one or more lone pairs, so that one migjit have considered it to be an L-type ligand. However, the use of a lone pair for bond formation would lead to a complex with an unpaired electron on the metal (.L —M). This electronic structure is less stable than that in which the unpaired electron and a metal elearon are paired to form the metal-ligand bond ( X- -M). It can be seen that in this case, all the electrons are paired, either as bonding pairs or as lone pairs. 

Ethylene behaves as an L-type ligand thanks to its doubly occupied TT orbital. It can transfer electron density to the metal (a donation interaction) by interaction with an empty orbital on the metal centre, whose symmetry is suitable (z, for example, 3-37a). This is a stabilizing two-electron interaction, which stabilizes the tt level of the ligand. Due to the relative energies of the two initial orbitals, the occupied MO is mainly concentrated on the ligand (a bonding orbital). 

To clarify these points, we shall consider the carbene as an L-type ligand (4-36a). It therefore acts as a r donor, using its lone pair described by the tier orbital, which interacts with an empty orbital on the metal (e.g. z, 4-38a). In this model, the Tip orbital is empty, so the carbene acquires a r-acceptor character (single face) (4-38b). The interaction scheme is similar to that in the Dewar-Chatt-Duncanson model (Chapter 3, 3.4.1) used, for example, to describe ethylene complexes or molecular hydrogen complexes ( 4.1.4). 

We now turn to an octahedral MLe complex with a electronic configuration, which we shall consider as the starting point for inorganic fragments. This complex obeys the 18-electron rule, just as methane obeys the octet rule. To be a Utde more concrete, though this choice is in no way unique, we shall consider a chromium complex [CrL ], with six neutral L-type ligands (PR3, CO, etc.), each of which supplies a pair of electrons to the metal, fn this complex, chromium is in the oxidation state zero, and the electronic configuration is indeed d. ... 

An L-function ligand is one which interacts with a metal center via a dative covalent bond (i.e. a coordinate bond), in which both electrons are donated by the L ligand. As such, an L-function ligand donates two electrons to a metal center. Since the metal uses no electrons in forming the M-L bond, an L-function ligand does not influence the valence of a metal center. Simple examples of L-type ligands include R3P, R2O, and CO, i.e. donor molecules that have lone pairs (Lewis bases). 

Sigma adducts are ligands that are bound to a metal center via a cr-bond pair (an L-type ligand). The cr-adduct can be a reaction intermediate that arises prior to an oxidative addition step or subsequent to a reductive elimination step. In this way, a cr-adduct is a midpoint between a free molecule or substrate and an activated molecule that then serves as... 

Davies et al. have developed a succinct set of guidelines to predict the most favorable position of nucleophilic attack on electronically saturated cationic metal complexes beating more than one unsaturated hydrocarbon ligand. In essence, the so-called DGM rules state that (i) polyenes (L -type ligands) are more reactive than polyenyls (L, -type ligands see also Scheme 11) (ii) open or acyclic ligands react before closed or cyclic ligands and (iii) addition to... 

Poorly backbonding metal -7c-donating R groups -L-type ligands -Electraphilic at carbon... 

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