Monodentate ligands amines

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

It thus came as a surprise that in the year 2000, three groups independently reported the use of three new classes of monodentate ligands (Scheme 28.2) [12], The ligands induced remarkably high enantioselectivities, comparable to those obtained using the best bidentate phosphines, in the rhodium-catalyzed enantioselective alkene hydrogenation. All three being based on a BINOL backbone, and devoid of chirality on phosphorus, these monophosphonites [13], monophosphites [14] and monophosphoramidites [15] are very easy to prepare and are equipped with a variable alkyl, alkoxy, or amine functionality, respectively. 

Complexes of monodentate aliphatic amine oxides with the lanthanides have not been extensively investigated. The only series of complexes with a ligand of this type, Et3NO, has been reported by Pavlenko et al. (181). The complexes have the formula [Ln(Et3N0)6][Cr(NCS)6]-2H20. 

The comparison of NHCs with various other monodentate ligands such as phosphines and amines on a [MoLsfCOls], a tra/w-[RhL2(CO)X], and various other [M(CO)nLm] complexes shows the significantly increased donor capacity relative to phosphines, even to trialkylphosphines (Table The... 

For the synthesis of bidentate ligands, supramolecular approaches have led to a renaissance in homogeneous catalyst discovery (Chapters 2, 4, 8, 9,10), and in a few cases even monodentate ligands have been modified in a supramolecular fashion (Chapter 8, Section 8.2). Combinations of monodentate ligands can be left to chance and in several instances this has led to successful, new catalysts [96]. Such heterocombinations can form spontaneously for steric or electronic reasons or the reactivity of the combinations can be different such that on certain occasions highly enantioselective catalysts are obtained. There are many ways to synthesize the desired heterocombinations selectively and the ionic modification outlined in Section 10.4 is only one of them since nitration (followed by reduction to amines) and sulfonation are robust methods, the ionic route may prove useful. Hydrogen bonding between different donor-acceptors (Chapters 2 and 8), Lewis add-base interactions (Chapter... 

Whilst what has been termed the real scientific advancement in this area took place in 1967,5 complexes of alkali and alkaline earth metal cations (M"+) with simple monodentate ligands can be traced back, through the metal-ammonias , to Faraday.10 It was not until almost a century later, however, that precise determinations of the stoichiometries of the M"+—NH3 products were realized and the term coordination was introduced to describe the bonding mode of the ligand.11"13 The alkali metals themselves were first noted to dissolve in liquid ammonia in 186314 and since that time it has been found that the metals also dissolve in amines and ethers.15... 

Reactions of Pt(O,0-MeCOCHCOMe)(MeCOCH2COMe)X (X = Cl, Br) with a wide variety of donors, including phosphines, arsines, amines, alkenes, diamines, etc. have been examined. In all cases the coordinated neutral acetylacetone is displaced. When monodentate ligands, L, are employed, products are of the type Pt(0,0-MeCOCHCOMe)XL. Bidentate ligands, LL, yield Pt(C-MeCOCHCOMe)X(LL) and bridging alkenes form PtCl(0,0-MeCOCHCOMe) (dialkene).494... 

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

Derivatives of Os3(CO)12 with monodentate ligands such as PR3, P(OR)3, RNC, RCN, NO, alkenes, and amines such as NH3, py, and NMe3 have been made. In most cases the basic structure of Os3(CO)i2 is maintained with six axial and six equatorial ligands. Some modifications are found when there is a reduction in coordination number on introducing three-electron donating NO. These compounds will be considered in Section III. 

Fe2(CO)9, and Fe3(CO)12, respectively. Similar disproportionations occurred with Ni(CO)4 and Co2(CO)8 which gave anionic species such as [Ni2(CO)6]2, [Ni3(CO)8]2, [Co(CO)4] , etc., upon treatment with ammonia or other amines. In contrast to the carbonyls of iron, nickel and cobalt, those of chromium, molybdenum and tungsten reacted with pyridine and 1,2-ethylenediamine to afford substitution products of the general composition M(CO)6 (py) (n = 1, 2, and 3) and M(CO)4(en) with the metal remaining in the oxidation state zero [25], Mainly as the result of this work, Hieber became convinced that the metal carbonyls should be regarded as true coordination compounds, and the coordinated CO should not be considered a radical but a monodentate ligand like NH3, pyridine, etc. He held this view despite the criticism by several of his contemporaries [3, 19] and was very pleased to see that in most textbooks published after 1940 this view had been accepted. 

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