In organometallic chemistry, an coordinatively unsaturated complex has fewer than 18 valence electrons and thus is susceptible to oxidative addition or coordination of an additional ligand. Unsaturation is characteristic of many catalysts. The opposite of coordinatively unsaturated is coordinatively saturated. Complexes that are coordinatively saturated rarely exhibit catalytic properties.
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Borinium ions have the formula [BX2]+, where X− is usual a bulky amide (R2N−). They have linear geometry at boron and are coordinatively unsaturated.
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In the migratory insertion, a ligand that is viewed as an anion (X) ligand in and a ligand that is viewed as neutral couple, generating a new anionic ligand. The anion and neutral ligands that react are adjacent. If the precursor complex is coordinatively saturated, migratory insertion often result in a coordinatively unsaturated product. A new (neutral) ligand can then react with the metal leading to a further insertion.
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Boronium ions have the formula [L2BR2]+ (L = Lewis base). Boronium ions are tetrahedral and coordinatively saturated. A well-known example is [(H3N)2BH2]+. Reaction of diborane with ammonia mainly gives [H2B(NH3)2]+(BH4)−.
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The parent compound ruthenocene is unreactive because it is coordinatively saturated and contains no reactive groups. Shvo's catalyst ([Ph4(η5-C4CO)]2H]}Ru2(CO)4(μ-H)) is also coordinatively saturated, but features reactive OH and RuH groups that enable it to function in transfer hydrogenation. It is used in hydrogenation of aldehydes, ketones, via transfer hydrogenation, in disproportionation of aldehydes to esters and in the isomerization of allylic alcohols. Chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium features a reactive chloro group, which is readily substituted by organic substrates.
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Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry. The entropy of activation is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate determining step.
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Depending on its steric bulk, DIP ligands form complexes of 2:1 and 1:1 ratios, M(DIP)Lx and M(DIP)2, respectively. The 2:1 complexes occur for unhindered DIP ligands. Although such complexes are coordinatively saturated, they have been studied for their electronic and structural properties. Formation of 2:1 complexation is suppressed with bulky DIP ligands.
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According to single crystal X-ray diffraction the compound adopts a slightly distorted square planar structure. In analyzing the bonding, it is a complex of Rh(I), a d8 transition metal ion. From the perspective of the 18-electron rule, the four ligands each provides two electrons, for a total of 16-electrons. As such the compound is coordinatively unsaturated, i.e.
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The hexahydrate consists of octahedral [Al(H2O)6]3+ centers and chloride counterions. Hydrogen bonds link the cation and anions. The hydrated form of aluminium chloride has an octahedral molecular geometry, with the central aluminum ion surrounded by six water ligand molecules. Being coordinatively saturated, the hydrate is of little value as a catalyst in Friedel-Crafts alkylation and related reactions.
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One water molecules is coordinated to each of metal center at the axial position. Paddlewheel unit (secondary building unit) of the HKUST-1 structure in the dehydrated state. The axial positions at the metal centers are not occupied (= coordinatively unsaturated site, CUS). HKUST-1 (HKUST ⇒ Hong Kong University of Science and Technology), which is also called MOF-199, is a material in the class of metal-organic frameworks (MOFs).
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Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes with an unsaturated coordination sphere. These special anions are essential components of homogeneous olefin polymerisation catalysts, where the active catalyst is a coordinatively unsaturated, cationic transition metal complex.
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Studies on Vaska's complex helped provide the conceptual framework for homogeneous catalysis. Vaska's complex, with 16 valence electrons, is considered "coordinatively unsaturated" and can thus bind to one two-electron or two one-electron ligands to become electronically saturated with 18 valence electrons. The addition of two one- electron ligands is called oxidative addition. Upon oxidative addition, the oxidation state of the iridium increases from Ir(I) to Ir(III).
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Unsaturated monomers are those having carbon–carbon double bonds. In general, the term "unsaturated" refers to the presence of one or more double (or triple) bonds and the ability to "saturate" the molecule by addition of H2. Some examples of unsaturated monomers include: acrylic acid, acrylamide, acryloyl chloride, and methyl methacrylate. Research suggests that unsaturated monomers that are coordinatively complexed together may be important in the process of enantioselective cyclopropanation of synthetic fibers.
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In the first reaction step, one hydrogen is added, with the other, coordinatively unsaturated, carbon being attached to the catalyst. The second step is the addition of hydrogen to the remaining carbon, producing a saturated fatty acid. The first step is reversible, such that the hydrogen is readsorbed on the catalyst and the double bond is re-formed. The intermediate with only one hydrogen added contains no double bond and can freely rotate.
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The effect was discovered when he noticed that a small amount of water enhanced the polymerizing activity in the Ziegler–Natta system and deduced that water must react with trimethylaluminum to give MAO. MAO serves multiple functions in the activation process. First it alkylates the metal-chloride pre-catalyst species giving Ti/Zr-methyl intermediates. Second, it abstracts a ligand from the methylated precatalysts, forming an electrophilic, coordinatively unsaturated catalysts that can undergo ethylene insertion.
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Metal carbonyl hydride are used as catalysts in the hydroformylation of olefins. Under industrial conditions the catalyst is usually formed in situ in a reaction of a metal salt precursor with the syngas. The hydroformylation starts with the generation of a coordinatively unsaturated 16-electron metal carbonyl hydride complex like HCo(CO)3 or HRh(CO)(PPh3)2 by dissociation of a ligand. Such complexes bind olefins in a first step via π-complexation.
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One water molecules is coordinated to each of the two metal centers at the axial position of the paddlewheel unit in the hydrated state, which is usually found if the material is handled in air. After an activation process (heating, vacuum), these water molecules can be removed (dehydrated state) and the coordination site at the metal atoms is left unoccupied. This unoccupied coordination site is called coordinatively unsaturated site (CUS) and can be accessed by other molecules.
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Agostic interaction is a term in organometallic chemistry for the interaction of a coordinatively-unsaturated transition metal with a C−H bond, when the two electrons involved in the C−H bond enter the empty d-orbital of a transition metal, resulting in a three-center two-electron bond. Many catalytic transformations, e.g. oxidative addition and reductive elimination, are proposed to proceed via intermediates featuring agostic interactions. Agostic interactions are observed throughout organometallic chemistry in alkyl, alkylidene, and polyenyl ligands.
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It was not until the 1980s that the reason was found: the zinc compound does not undergo the beta-hydride elimination reaction whereas the compound of the transition metal copper does so. Alkyl and aryl zinc compounds are contain the linear C—Zn—C motif. Because the zinc centre is coordinatively unsaturated, the compounds are powerful electrophiles. In fact the low-molecular weight compounds will ignite spontaneously on contact with air and are immediately destroyed by reaction with water molecules.
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Associative pathways are characterized by binding of the attacking nucleophile to give a discrete, detectable intermediate followed by loss of another ligand. Complexes that undergo associative substitution are either coordinatively unsaturated or contain a ligand that can change its bonding to the metal, e.g. change in hapticity or bending of a nitrogen oxide ligand (NO). In homogeneous catalysis, the associative pathway is desirable because the binding event, and hence the selectivity of the reaction, depends not only on the nature of the metal catalyst but also on the substrate.
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For example, with Wilkinson's catalyst, one triphenylphosphine ligand must dissociate to give the coordinatively unsaturated 14-electron species which can participate in the catalytic cycle: :Wilkinson's catalyst requires activation before it can participate in the catalytic cycle Similarly, for an autocatalytic reaction, where one of the reaction products catalyzes the reaction itself, the rate of reaction is low initially until sufficient products have formed to catalyze the reaction. Reactions generally accelerate when heat is applied. Where a reaction is exothermic, the rate of the reaction may initially be low. As the reaction proceeds, heat is generated, and the rate of reaction increases.
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Some metal alkyls feature agostic interactions between a C-H bond on the alkyl group and the metal. Such interactions are especially common for complexes of early transition metals in their highest oxidation states. One determinant of the kinetic stability of metal-alkyl complexes is the presence of hydrogen at the position beta to the metal. If such hydrogens are present and if the metal center is coordinatively unsaturated, then the complex can undergo beta-hydride elimination to form a metal-alkene complex: center These conversions are assumed to proceed via the intermediacy of agostic interactions.
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The development of well- defined phosphine- or NHC-ligated gold(I) complexes was an important advance and led to significant increase in interest in the synthetic applications of gold catalysis. Ligated gold(I) complexes are typically prepared and stored as the bench-stable (but unreactive) chlorides, LAuCl, e.g., chloro(triphenylphosphine)gold(I), which are typically activated via halide abstraction with silver salts like AgOTf, AgBF4, or AgSbF6 to generate a cationic gold(I) species. Although the coordinatively unsaturated complex "LAu+" is notionally generated from a LAuCl/AgX mixture, the exact nature of the cationic gold species and the role of the silver salt remains somewhat contentious.
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As shown in Eq. 2, the neutral pathway of the Heck reaction begins with the oxidative addition of the aryl or alkenyl halide into a coordinatively unsaturated palladium(0) complex (typically bound to two phosphine ligands) to give complex I. Dissociation of a phosphine ligand followed by association of the alkene yields complex II, and migratory insertion of the alkene into the carbon-palladium bond establishes the key carbon-carbon bond. Insertion takes place in a suprafacial fashion, but the dihedral angle between the alkene and palladium-carbon bond during insertion can vary from 0° to ~90°. After insertion, β-hydride elimination affords the product and a palladium(II)-hydrido complex IV, which is reduced by base back to palladium(0).Amatore, C.; Azzabi, M.; Jutand, A. J. Am. Chem. Soc.
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The term agostic, derived from the Ancient Greek word for "to hold close to oneself", was coined by Maurice Brookhart and Malcolm Green, on the suggestion of the classicist Jasper Griffin, to describe this and many other interactions between a transition metal and a C−H bond. Often such agostic interactions involve alkyl or aryl groups that are held close to the metal center through an additional σ-bond.. Short interactions between hydrocarbon substituents and coordinatively unsaturated metal complexes have been noted since the 1960s. For example, in tris(triphenylphosphine) ruthenium dichloride, a short interaction is observed between the ruthenium(II) center and a hydrogen atom on the ortho position of one of the nine phenyl rings. Complexes of borohydride are described as using the three-center two-electron bonding model.
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C60 forms complexes akin to the more common alkenes. Complexes have been reported molybdenum, tungsten, platinum, palladium, iridium, and titanium. The pentacarbonyl species are produced by photochemical reactions. : M(CO)6 \+ C60 → M(η2-C60)(CO)5 \+ CO (M = Mo, W) In the case of platinum complex, the labile ethylene ligand is the leaving group in a thermal reaction: : Pt(η2-C2H4)(PPh3)2 \+ C60 → Pt(η2-C60)(PPh3)2 \+ C2H4 Titanocene complexes have also been reported: : (η5-Cp)2Ti(η2-(CH3)3SiC≡CSi(CH3)3) + C60 → (η5-Cp)2Ti(η2-C60) + (CH3)3SiC≡CSi(CH3)3 Coordinatively unsaturated precursors, such as Vaska's complex, for adducts with C60: : trans-Ir(CO)Cl(PPh3)2 \+ C60 → Ir(CO)Cl(η2-C60)(PPh3)2 One such iridium complex, [Ir(η2-C60)(CO)Cl(Ph2CH2C6H4OCH2Ph)2] has been prepared where the metal center projects two electron-rich 'arms' that embrace the C60 guest.
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Puddephatt has conducted pioneering research on synthesis, reactivity and elucidation of mechanisms in the organometallic chemistry of the noble metals, particularly related to the role of organometallic compounds in catalysis and in materials science. He has elucidated the mechanisms of reactions which are fundamental in many homogenous catalytic processes, most notably in studies of oxidative addition and reductive elimination with alkylplatinum and alkyl–gold complexes and of skeletal rearrangements of metallacyclobutane complexes, often called the Puddephatt rearrangement. Remarkable coordinatively unsaturated platinum clusters and platinum–rhenium clusters have been synthesised and shown to be excellent mimics of the surface reactivity, related to heterogeneous catalysis by supported platinum and bimetallic platinum–rhenium catalysts. New, commercially useful organometallic precursors for chemical vapour deposition (CVD) of thin films of metals such as palladium, platinum, silver and gold have been discovered and the fundamental mechanisms of those CVD processes elucidated.
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The chemical properties of Ru3(CO)12 have been widely studied, and the cluster has been converted to hundreds of derivatives. High pressures of CO convert the cluster to the monomeric ruthenium pentacarbonyl, which reverts to the parent cluster upon standing. :Ru3(CO)12 \+ 3 CO 3 Ru(CO)5 Keq = 3.3 x 10−7 mol dm−3 at room temperature The instability of Ru(CO)5 contrasts with the robustness of the corresponding Fe(CO)5. The condensation of Ru(CO)5 into Ru3(CO)12 proceeds via initial, rate-limiting loss of CO to give the unstable, coordinatively unsaturated species Ru(CO)4. This tetracarbonyl binds Ru(CO)5, initiating the condensation.Hastings, W. R.; Roussel, M. R.; Baird, M. C. "Mechanism of the conversion of [Ru(CO)5] into [Ru3(CO)12]" Journal of the Chemical Society, Dalton Transactions, 1990, pages 203-205. Upon warming under a pressure of hydrogen, Ru3(CO)12 converts to the tetrahedral cluster H4Ru4(CO)12.Bruce, M. I.; Williams, M. L. "Dodecacarbonyl(tetrahydrido)tetraruthenium, Ru4(μ-H)4(CO)12" Inorganic Syntheses, 1989, volume 26, pages 262-63. .
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