Monodentate ligands, transition metal

Monodentate (monometallic monoconnective) phosphor-1,1-dithiolato ligands are rare. Bidentate (monometallic biconnective) form chelate rings and three sub-types can be distinguished according to the degree of asymmetry (Scheme 2). The most asymmetric type (anisobidentate) occurs when a covalent bond is associated with a secondary bond this takes place mostly in main-group metal complexes. The second type is rare and is the result of the association between a covalent and a dative coordinate bond. The symmetric bidentate bonding (isobidentate) is found mainly in transition metal complexes. 

Distibines react as monodentate (type 5) or bridging bidentate (type 6) ligands through donation of the lone pairs of electrons to main group or transition metal centers. Fission of the Sb-Sb bond leads to complexes with bridging R2Sb ligands (type 7, 8 and 9) (Scheme 3). 

In order to describe the different conformations in type 16-20 molecules, the torsion angles (Ep)M-Sb-Y-Sb (Ep = assumed direction of the lone pair at antimony M = transition metal Y = CH2, O, S) were used. The Ep-Sb-Y-Sb torsion angles, which are considered in complexes with monodentate (type 16) coordination of the Sb ligand, were calculated from the corresponding pairs of torsion angles C-Sb-Y-Sb (Scheme 8). 

Recently, two new P- and C-chiral monodentate phosphines 13 were reported. The ligands were applied in a number of transition metal-catalyzed reactions, though ee-values in the rhodium-catalyzed hydrogenation of N-acyl dehydrophenylalanine were only moderate [37]. 

N-Diazeniumdiolates can also undergo some reactions other than those directly associated with NO release. They act as bidentate chelators and form complexes with a variety of transition metal ions. Suitable choice of the ligands can lead to the formation of complexes in which the diazeniumdiolates act as a monodentate ligand coordinated to Cu(II) at 02 only [223]. 

For monodentate ligands, e.g., triphenylphosphane, Tolman s cone-angle 0 and the electronic parameter x have a significant influence on the activity and the selectivity of the resulting catalyst system [24,25]. As regards bidentate ligands, which provide two coordination centers for the transition metal, the so-called bite angle fi determines the selectivity of the formed aldehydes. 

Although, in principle, the chemistry here reported should be centered on the late transition metals, sometimes we will jump the frontier between late and middle or early transition metal since this line could be more or less diffuse and could change as a function of the history. At least seven different coordination modes have been identified (I-VII, Scheme 3) as the main bonding modes. In modes I and II the ylide behaves as neutral and monodentate, bonded exclusively through the Ca atom (kC mode) this is the case for simple ylides and carbodipho-sphoranes. Mode 111 covers the variants of a metallated ylide, that is, a situation in which the metal replaces a substituent of the ylide and transforms it into an anionic ligand. 

Aryl-5-methyl-I,2,4-oxadiazoles serve as monodentate ligands for a number of transition metal complexes <85SA(A)1433>. 

Acetylenes are discussed separately here because, in reactions with transition metal compounds, they can act in a variety of ways 34). Thus, they can act as monodentate (2-electron donors) or as bridging groups (4-electron donors), or they can undergo chemical transformations to form cyclobutadiene, cyclopentadienone, or other moieties that incorporate the parent acetylene as a part of the cyclic ir-ligand. Some examples of this last type have already been mentioned (Sections IV,C,l and 4). 

Cationic transition metal amide complexes have been investigated in part because of their potential in catalysis p irticularly for olefin polymerization. Much of this work has concerned polydentate amido, linked cyclopentadienyl-amido or delocalized nitrogen centred bidentate ligands (see later). However, the structures of a small number of cationic complexes containing monodentate amido ligands have been determined. These include... 

Reetz, M.T. and Li, X. (2005) The influence of mixtures of monodentate achiral ligands on the regioselectivity of transition-metal-catalyzed hydroformylation. Angew. Chem., Int. Ed., 44, 2962-2964. 

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