Imido ligand

Monomeric sulfur diimides have an extensive coordination chemistry as might be anticipated from the availability of three potential donor sites and two r-bonds. In addition, they are prone to fragmentation to produce thionitroso and, subsequently, sulfido and imido ligands. Under mild conditions with suitable coordinatively unsaturated metal... 

Unique examples of N,A/ , N "-tri(isopropyl)guanidinate complexes of Nb(V) and Ta(V) bearing additional terminal imido ligands have been obtained as depicted in Scheme 116. Depending on the reaction conditions, these reactions may also lead to the formation of compounds containing dianionic guanidinate... 

Over the last decade a number of high oxidation state ruthenium porphyrin complexes containing 0x0 or imido ligands have been reported and have been thoroughly studied for their role in oxidation and atom-transfer chemistry. Although comparisons can be drawn with organometallic species (carbene, imido. and 0x0 ligands are formally isolobal) the chemistry of the 0x0 and imido complexes is beyond the scope of the review and will not be covered here. 

Besides, iron(IV) imides have also been implicated as reaction intermediates in nitrogen atom/group transfer reactions [36]. The first structurally characterized iron-imide complex was reported by Lee and coworkers [37] through the reaction of FeCls with 2 equiv. of LiNH Bu in THF, giving one of the products as a stable site-differentiated cubane having three Fe(lll) and one Fe(IV) centers and the Fe(IV) center has a terminally bonded imido ligand (Scheme 4). 

Complexes with monodentate amino, amido, and imido ligands 284... 

The primary reason for attempting to synthesize imido alkylidene complexes, e.g., W(NR)(CH-f-Bu)(0-f-Bu)2, was the belief that the appropriate imido ligand will block bimolecular decomposition reactions more effectively than an oxo... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

A variety of catalytically active five-coordinate tungsten oxo and imido alkylidene complexes also have been prepared that contain some donor amine or pyridine linked either to the imido ligand or to a phenyl ligand bound to the metal (A, Scheme 2) [101-105]. Such species show metathesis activity (e.g., ROMP of norbornene),but there does not appear to be any proof that the integrity of the initiator is maintained. 

In rhenium compounds that contain two metal-imido ligands, these tend not to occupy positions trans to one another for the same reason. A trigonal-bipyramidal geometry has been identified in 2 and 3, even to the extent that the potentially chelating ligand Ph2PCH2PPh2 acts as a monodentate ligand. 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

Addition of Br0nsted acids to double bonds involving a metal center is well known in molecular chemistry [15]. Therefore, it was expected to take place when complexes bearing an alkylidene or an imido ligand were allowed to react with acidic... 

OSiHs) [82] found that the unsymmetrical catalysts (X Y) are systematically more efficient for all systems (W, Mo and Re). Overall, Mo and W catalysts are more efficient due to the presence of the imido ligand, because it favors distortion of the initial complexes, except when X Y-in this case, all systems are equally efficient [81],... 

Recent developments have been reported for imido-molybdenum systems. When X = 2,5-dimethylpyrrolyl, more stable surface species can be obtained (although a second minor species, formed by the addition of a surface hydroxyl group to the alkylidene ligand, is also present). Moreover, by tuning the imido ligand, the catalytic activity has been improved dramatically (Table 11.3, bottom rows) [20]. 

The Tc complex [TcI(N—Ar)s] (28) (see Section 5.2.2.1.2) can be reduced with Na° in THE to yield the green, nonbridged, dinuclear compound [Tc2(NAr)6] (46), in which three imido-ligands are bound to the Tc center and connected by a single bond to the second technetium. The molecule has a staggered, ethane-like structure and is diamagnetic. Reduction of (28) yields another homoleptic imido-complex of Tc, the imido-bridged, tetrahedral, dinuclear compound [Tc2(/u-NAr )2(NAr )4] (47) (Ar = 2,6-diisopropylphenyl). The conformation could be confirmed by X-ray structure analysis the assumption of a Tc Tc bond is confirmed by the... 

Dianionic organoimido ligands (RN ) are isoelectronic with the oxo group, but complexes of technetium-containing imido ligands are comparatively rare. Although not yet employed in... 

Imido-ligands, NR, are isoelectronic with the oxo ligand and should produce complexes of similar types. However, there are two important differences between NR and The imido-group has better electron-donating capabilities and the fact that with the substituent R the steric requirements of this ligand can be controlled within a wide range. This allows the stabilization of imido species with low coordination numbers and with ligands which are easily oxidized. 

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