Organic ligand

Organic ligands play a pivotal role in the alteration of the intrinsic catalytic properties of metals used in hydroformylation. Such ligands have a dramatic influence on the reactivity as well as chemo-, regio-, and stereoselective discrimination of the catalyst. Their steric and electronic construction and concentrations in relation to the metal used are decisive for the success of the transformation. Therefore, occasionally they are termed co-catalysts. 

The first investigations in the framework of hydroformylation by Otto Roelen and successors were based on ligand-free systems. Of course, such catalysts are not constituted by the naked metal. They host, besides H and CO, diolefins, carboxylates, or halogenides, which are replaced either in the beginning or throughout the hydroformylation mechanism by solvent molecules, substrates, or reagents. Such unmodified catalysts are still in use and give excellent results in numerous applications. 

Phosphines (also called phosphanes) are usually characterized by three carbon atoms surrounding the central phosphorus atom. Exceptions are unsaturated P-heterocycles and primary or secondary phosphines the latter do not play a role as ligands up to now. 

Hydroformylation Fundamentals, Processes, and Applications in Organic Synthesis, First Edition. 

SPO = secondary phosphine oxide HASPO = heteroatom substituted phosphine oxide 

Interaction with Organic Ligands A. Monocarboxylate Ligands 

Published equilibrium constants for monocarboxylato complexes are summarized in Table VII. All that can be deduced with certainty from these data is that the anions derived from monocarboxylic acids form rather weak complexes with beryllium. In all probability they act as monodentate ligands. The possibility of bidentate chelation using both carboxylate oxygen atoms can be ruled out on the grounds 

A different situation obtains in compounds such as the basic beryllium monocarboxylates (140-142). In particular the acetate derivative, Be40(02CCH3)6, has been known in the solid state for nearly 100 years (143). A partial structure of this compound is in Fig. 13 (144). The central oxide ion is surrounded by a tetrahedron of beryllium atoms. The acetate ions form bridges across the six edges of the Be4 tetrahedron, with each acetate bonding with two Be atoms. Each berylium atom in surrounded by an approximate tetrahedron of 

An analogous nitrato complex, Be40(N03)e, is known in the solid state 145). The presence of a central 4-coordinate oxide ion and bridging nitrato ions has been confirmed by X-ray methods 146). The structures of these solid-state JU.4-0X0 complexes lend support to the /jLs-oxo structures suggested for phosphate and carbonate complexes in solution (see Section IV). 

In all cases studied the standard enthalpy change accompanying the replacement of two water molecules by the chelating ligand is 

Ligand Method T/°C Medium Log of equilibrium constants, remarks Ref. 

Doudoroff, G. Leduc, and C. R. Schneider, Acute Toxicity to Fish of Solutions Containing Complex Metal Cyanides, in Relation to Concentrations of Molecular Hydrocyanic Acid, Trans, Amer, Fish Soc., 95 6-22 (1966). 

Second, a wide variety of organic compounds in natural waters and wastewater can act as complexing agents for metal ions. The nature and extent of metal ion complexation by natural water organics is not well-defined, probably because of the poorly defined nature of these organic compounds and also because of the staggering complexity of these 

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If a catalyst is to work well in solution, it (and tire reactants) must be sufficiently soluble and stable. Most polar catalysts (e.g., acids and bases) are used in water and most organometallic catalysts (compounds of metals witli organic ligands bonded to tliem) are used in organic solvents. Some enzymes function in aqueous biological solutions, witli tlieir solubilities detennined by the polar functional groups (R groups) on tlieir outer surfaces. 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

In the case of complex entities such as organic ligands (particularly if they are substituted) the multiplying prefixes bis-, tris-, tetrakis-, pentakis-,. . . are used, i.e.,-kis is added starting from tetra-. The modified entity is often placed within parentheses to avoid ambiguity. 

TABLE 8.13 Cumulative Formation Constants for Metal Complexes with Organic Ligands... 

Inorganic complexes confaining organic ligands exhibif af leasf some vibrations which are characteristic of fhe ligands. 

The lanthanides form many compounds with organic ligands. Some of these compounds ate water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating agents). The complex formation is pH-dependent. Oil-soluble compounds ate used extensively in the industrial separation of rate earths by tiquid—tiquid extraction. The preferred extractants ate catboxyhc acids, otganophosphoms acids and esters, and tetraaLkylammonium salts. 

Pu(IV) forms polyatomic complexes with inorganic and organic ligands. As the number of anionic ligands increases, cationic, neutral, and anionic complexes form and the sequential stabiUty constants, typically decrease. For the following reaction, where M is Pu + and L is a ligand,... 

The need for simple descriptions of complicated organic ligands has led to the evolution of some trivial nomenclature systems, such as those for crown ethers (e.g. 76) 72AG(E)16) and cryptands 73MI10200), which have become quite elaborate 8OMII0200). These systems are intended primarily to indicate topology, and the positions of potential donor atoms, and are not particularly appropriate for general use. 

Ti in [Ti(> 2-Cl04)4] and Ni" in [Ni(>j -C104)L2] where L is a chiral bidenlate organic ligand.Sometimes both and modes occur in the same compound. The biden-latc bridging mode occurs in the silver complex [Ag /x,>j -00(0)20- (m-xylene)2]- The structure of appropriate segments of some of these compounds arc in Fig. 17.23. The distinction between coordinated and non-coordinaied ( ionic ) perchlorate is sometimes hard to make and there is an almost continuous... 

L. G. SiLLfiN and A. E. Martell, Stability Constants of Metal-ion Complexes, The Chemical Society, London, Special Publications No. 17, 1964, 754 pp., and No. 25, 1971, 865 pp. Stability Constants of Metal-lon Complexes, Part A. Inorganic Ligands (E. Hcigfeldt, ed.), 1982, pp. 310, Part B. Organic Ligands (D. Perrin, ed.), 1979, pp. 1263. Pergamon Press, Oxford. A continually updated database is now provided by L. D. Pettit and K. J. Powell (eds.), IVPAC Stability Constants Database, lUPAC and Academic Software. 

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