Monodentate donors

Thiolates (RS ) represent an extensive family of ligands, and include chelating forms. Thiolates are known to act as monodentate donors, but often act in a bridging role. There is a clear biological interest, through participation of thiolates (cysteine residues) as donors in many metalloproteins both as terminal S donors and bridging ligands in, for example, Fe S clusters. 

As documented in detail for organolithium species, ligand and donor play a key role in determining the degree of aggregation. Methyllithium adopts a hexameric structure in hydrocarbon solvents.13,15 In the presence of monodentate, donors such as THF or diethyl ether tetramers are observed, while the increase in donor denticity to 2 (1,1-Dimethoxyethane (DME), N,N,N, N -Tetramethylethylenediamine (TMEDA)) affords monomeric structures. Further documenting the differences between solution and solid states, [CH3Li]4 adopts a tetrameric structure in the latter.15,15a-15c... 

Diorganozinc Compounds Bearing Monodentate Donor Ligands 332... 

Table 42 Neutral Adducts of the Niobium and Tantalum Trihalides with Monodentate Donors...

Ni(acac)2 reacts with a variety of monodentate donors giving mono and bis adducts Ni(acac)2B (n = 1,2 B = H20, primary and secondary amines, pyridine and substituted pyridines, pyridine iV-oxide, alcohols, dioxane, substituted benzaldehydes).1558,1563-1570 Details of the structures of some complexes are reported in Table 78. The chelate ring of the coordinated /3-diketones is nearly planar, and, in thl mononuclear complexes, the Ni—O bond distances (as well as the C—O and C—C bond distances within the chelate ring) are substantially similar. Two different dinuclear structures have been found in the two complexes Ni2(acac)4B [B = py (197),1540,1571,1530 Ph3AsO (198)1542,1572]. 

Imidazoles act as monodentate donors for palladium, forming complexes [PdC LJ and [PdLJCfe.216"221 Both cis and trans isomers of the former complexes have been isolated and their antitumour properties investigated.216 A polymeric compound [Pd(imidazolato)2] has also been isolated which contains bidentate bridging imidazolato ligands. 

Alkylphosphonate groups are monodentate donors of intermediate hardness that may replace carboxylate groups in chelating agents such as nitrilotriacetic acid (NTA), ethylenediaminetetra-acetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA). The corresponding ligands nitrilotrimethylenephosphonic acid (NTP), 1, ethylenediaminetetramethylenephosphonic acid (EDTP),... 

In the [M(CO)2L2b -diene)] complexes, the monodentate donor ligands are found almost exclusively at the a and / positions. There is evidence for the ad and ae stereoisomers only for [Cr(CO)2 P(CH3)3 P(OCH3)3 -(i/ -butadiene)] (36a) (29). A hindered ligand movement, which causes temperature-dependent NMR signals for the donor ligands, is explained by a... 

The crystal structure determinations and spectral data show that the carbonyl oxygen in the 1 1 adducts of trichlorides of the type 329 (R = Aik, Ar) with monodentate donors like pyridine is coordinated intramolecularly to tin723,743 -746. However, the intramolecular coordination can be broken in the 1 2 adducts with monodentate donors and with 1 1 bidentate donors like bipyridine. 

Central to the stability and chemistry of complexes formed by mixed donor ligands are two key concepts of coordination chemistry. The first is the chelate effect, which applies to all polydentate ligands, and reflects the increase in stability of a type of complex as monodentate donor molecnles are replaced by polydentates with donors linked by chelate rings. The hard-soft acid-base theory is particnlarly relevant to mixed donor ligands where donors of distinctly different character may bind to a central metal ion. The like prefers like concept means hard nonpolarizable donor atoms (N and O, for example) bond preferentially to hard nonpolarizable metal... 

Phosphine and arsine oxides, and other neutral monodentate donors... 

A motif found in the majority of alkali metal stabilized carbanion crystal structures is a nearly planar four-membered ring (13) with two metal atoms (M ) and two anions (A ), i.e. dimer. This simple pattern is rarely observed unadorned as in (13), yet almost every alkali metal and alkaline earth carbanion aggregate can be built up from this basic unit The simplest possible embellishment to (13) is addition of two substituents (S) which produces a planar aggregate (14). Typically the substituents (S) in (14) are solvent molecules with heteroatoms that serve to donate a lone pair of electrons to the metal (M). Only slightly more complex than (14) is the four coordinate metal dimer (15). Often the substiments (S) in (15) are joined by a linear chain. The most common of these chains are tetramethylethylenediamine (TMEDA) or dimethoxyethane (DME) so that the spirocyclic structure (16) ensues. Alternatively the donors (S) in (16) have been observed as halide anions (X ) when the metal (M ) is a divalent cation, e.g. (17) or (18). Obviously, the chelate rings found in (16) are entropically favorable relative to monodentate donors (S) in (14), (15), (17) or (18) (Scheme 2). 

In your reaction, the aldehyde R group is aromatic (2). Aromatic R groups produce more stable imine products than do alkyl R substituents. Salpd is a tetradentate ligand it contains four atoms, two N and two O, that can readily bind to a metal ion. Metal complexes containing multidentate ligands show enhanced stability over those with analogous monodentate donors. Bjerrum first noted the enhanced stability of five-membered rings... 

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