Unsaturated metal center , coordinatively

The anion dissociates, and the coordinatively unsaturated metal center then picks up a monomer molecule for subsequent enchainment. This dissociative model has been favored in the past [16, 21-23, 27-28] since it allows a convenient explanation of the observed polymer stereochemistry by considering only the roles of the ligand and the alkyl chain in the cationic metallocene complex. However, anion dissociation opposes the electrostatic attraction between cation and anion and is therefore energetically expensive. So does it operate at all ... 

Since the first discovery of transition metal allenylidene complexes (M=G=C=C<) in 1976, " these complexes have attracted a great deal of attention as a new type of organometallic intermediates. Among a variety of such complexes, cationic ruthenium allenylidene complexes Ru =C=C=GR R, readily available by dehydration of propargylic alcohols coordinated to an unsaturated metal center, can be regarded as stabilized propargylic cation equivalents because of the extensive contribution of the ruthenium-alkynyl resonance form... 

The other approach, that is followed here, involves proposing a hypothetical mechanism for the homogeneously catalyzed process using only steps for which there is good precedent in organo-transition metal chemistry and involves choosing a system to optimize those steps that appear to be potentially the most troublesome. A reasonable pathway for reducing CO to methanol is shown in Scheme 1. Steps (a) and (d) show oxidative addition of H2 to a coordinatively unsaturated metal center, a well-known process (5). Step (b) is the insertion of... 

A number of readily reversible cr-7r rearrangements have been observed wherein a labile ligand such as carbon monoxide is lost by pyrolysis or photolysis, producing a coordinatively unsaturated metal center, which then regains coordinative saturation by means of a tr-n rearrangement. For example, irradiation of o--alkyl-7r-cyclopentadienyl-molybdenum tricarbonyl (15) produces the rr-allene complex (16) (25). These... 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

Since this initial report, NHCs have been used to stabilize a number of additional high-valent metal complexes bearing oxo and nitrido ligands (Chart 1) [26-32]. In contrast to the MTO example, these complexes and the high-valent metal precursors employed in their synthesis are not especially strong oxidants. Consequently, the preparation of these complexes often can be achieved by simple addition of the NHC to an unsaturated metal center or via displacement of a weakly coordinated solvent molecule such... 

Thiolate donors in [M(S )] fragments with coordinatively unsaturated metal centers may stabilize these metal centers by either n donation or formation of M—S—M bridges. 

Although other transition metals have been found to catalyze hydroborations with HBcat, early studies have shown that rhodium complexes are the most effective for reactions of simple alkenes. The catalytic cycle proposed resembles one suggested previously for the rhodium-catalyzed addition of carborane B-H bonds to the C=C unit in acrylate esters. The reaction is believed to proceed via initial loss of phosphine and oxidative addition (see Oxidative Addition) of the B-H bond of HBcat to the coordinatively unsaturated (see Coordinative Saturation Unsaturation) rhodium center. Coordination ofthe alkene (seeAlkene Complexes) and subsequent insertion (see Insertion) into the Rh-H bond and reductive elimination (see Reductive Elimination) of the B-C bond then generates the organoboronate ester product(s) (Scheme 1). 

A more recent addition to the half-sandwich chemistry of ruthenium is given by a number of complexes where the central metal obeys a 16 valence electron count. These coordinatively unsaturated (see Coordinative Saturation Unsaturation) metal centers are widely invoked as intermediates or transition states in dissociative substitution processes or in catalytic transformations at transition metal centers. Such complexes are not, however, easily isolated. The most common way to stabilize such species is by coordinating sterically bulky ligands to the metal, thereby preventing further ligand addition. They can also be stabilized in the form of dimeric complexes. 

Optimal substrates for a synergistic [15,16] inner-sphere [5,12] interaction with the electronically ambivalent species L M-R (R=H, alkyl, aryl) have a similarly ambivalent acceptor/donor make-up They should have electron pair-donating heteroatoms such as O, N or S in order to form coordinative bonds to the coordinatively unsaturated metal center of the organometallic compound (Eq. (1)), at the same time, a low-lying unoccupied orbital such as the LUMO (w ) of a 7t system is required for the acceptor function. 

The silyl amide type ligands have been used extensively in rare earth chemistry, as well as in actinide and transition metal chemistry, to stabilize electronically unsaturated metal centers due to the available lone pair on the nitrogen donor atom. Because of the relatively larger steric encumbrance, the rare earth complexes with silyl amide type ligands often exhibit low coordination numbers. As a consequence, the large and electropositive rare earth metal centers are accessible to external reagents, which make them more active in many reactions. 

Introduction of Si-H functionality at the y-silicon of these metallo-siloxanes promises a rich area concerning consecutive reactions, especially oxidative addition to coordinatively unsatured metal centers [3],... 

One model for bonding in a diazo compound would be die ylide (89 equation 37)." Unlike many other ylides, diazoalkanes are stable to air and water. With acid, however, protonation can lead to the highly reactive salt (90), the functional equivalent of the corresponding carbocation. As the substituents on the diazo group are made increasingly electron withdrawing, the ylide becomes less basic, and thus more stable to acid. Reaction of a diazo compound with a transition metal can also often be understood as proceeding via initial donation of electron density by (89) to a coordinatively unsaturated metal center. 

Hydrocyanation of styrene 26 (eq. (7)) has been examined in some detail. With Ni[P(0-o-tolyl)3]4 27 the branched nitrile 29 is strongly favored over the linear one, which is explained by the intermediary formation of a detectable alkyl species 28. The stability of this intermediate is attributed to the donation of aromatic ring electrons to the coordinatively unsaturated metal center. Crystal structures of related compounds are reported in the literature [53, 54]. 

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