Unsaturation, coordinative

Fligh reactivity may also result from easy displacement of weakly bound ligands. The zirconium compound [Cp2Zr(THF)(CH3)]+, shown in Fig. 2.1 and discussed in the previous section, is an example. Unlike RhCl(PPh3)3, which tries to form extra bonds in its reactions, the zirconium compound s reactivity in a catalytic reaction is due to the easy displacement of THF by the substrate. 

The term coordinative unsaturation includes this type of reactivity also. In other words, the ability to form extra bonds or facile displacement of weakly bound ligands, which in many cases may just be solvent molecules, are both manifestations of coordinative unsaturation. 

A quantitative estimation of the steric demand of L can be made in terms of its cone angle. As shown in Fig. 2.2, it is the angle of a cone with its vertex at the metal atom and a metal-phosphorus distance of about 22.8 nm. The cone is created by the surface that just encloses all the ligand atoms for all orientations resulting from the rotation around the metal-phosphorus bond. 

Finally, many complexes that participate in homogeneous catalytic reactions have electron counts less than 16. This is especially true for high-oxidation-state early-transition-metal complexes such as (C2H5)TiCl3, Ti(OPr )4, etc. Cat-alytically active, late-transition-element complexes with electron counts less than sixteen are also known. An important example is RhCl(PPh3)2, a 14-electron complex that plays a crucial role in homogeneous hydrogenation reactions (see Section 7.3.1). 

Organometalhc complexes where the metal is in a low oxidation state tend to be reactive if the electron count is less than 18. They undergo reactions to form extra bonds so that an electron count of 18 is reached. When the electron count is less than 18, the metal complex is often classified as electronicallyuDsatarsted. Thus complexes such as [RhCKPPhj) ], [Ru(CHPh)Cl2(PCy3)2], and [Ni(C3H5)J are electronically unsaturated, while [RhH(CO)(PPh3)3] and [Co(CO)J are saturated. 

Electronic unsaturation is not identical to coordinative unsaturation. Coordinative unsaturation basically means whether or not one of the reactants in a catalytic reaction can easily coordinate to the metal 

Irrespective of the electron count, coordinative unsaturation may result from easy dissociation of a ligand. The zirconium complex 2.61 and the rhodium complex 2.59 illustrate this point. The zirconium compound is electronically unsaturated, but its reactivity in catalytic alkene polymerization is due to the easy displacement of THF by alkene. Complex 2.59 is electronically saturated but undergoes PPhj dissociation to generate coordinative and electronic unsaturation. 

Coordinative unsaturation can sometimes be induced by bulky ligands. A few such ligands can take up most of the space around the metal atom and prevent the presence of a full complement of ligands. As an example, consider the nickel complex 2.28, which has an electron count of 18. Steric repulsion between the ligands causes ligand dissociation in solution, and the equilibrium of (2.2.3.1) is established. The species NiLj (L = PPhj) on the right-hand side of the equilibrium has an electron count of 16. It is electronically and coordinatively unsaturated. 

Cataljdically active, late transition metal complexes with electron counts less than 16 are also known. Examples are RhCXPPhj), PdCPBUj ), and [PdtPhjCPBUj jBr] (see 2.29 and 2.36). These are 14-electron complexes and, as we will see later, take part in many homogeneous catalytic reactions. 

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Attention should be paid to the fact that the ratio of Pd and phosphine ligand in active catalysts is crucial for determining the reaction paths. It is believed that dba is displaced completely with phosphines when Pd2(dba)3 is mixed with phosphines in solution. However the displacement is not eom-plcte[16]. Also, it should be considered that dba itself is a monodentate alkene ligand, and it may inhibit the coordination of a sterically hindered olefinic bond in substrates. In such a case, no reaction takes place, and it is recommended to prepare Pd(0) catalysts by the reaction of Pd(OAc)2 with a definite amount of phosphinesflO]. In this way a coordinatively unsaturated Pd(0) catalyst can be generated. Preparation of Pd3(tbaa)3 tbaa == tribenzylidene-acetylacetone) was reported[17], but the complex actually obtained was Pd(dba)2[l8],... 

Another reaction in the last step is the syn elimination ofhydrogen with Pd as H—Pd—X, which takes place with alkyl Pd complexes, and the Pd hydride and an alkene are formed. The insertion of an alkene into Pd hydride and the elimination of, (3-hydrogen are reversible steps. The elimination of, 3-hydrogen generates the alkene, and both the hydrogen and the alkene coordinate to Pd, increasing the coordination number of Pd by one. Therefore, the / -elimination requires coordinative unsaturation on Pd complexes. The, 3-hydrogen eliminated should be syn to Pd. 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Thermodynamic properties (71,72), force constants (73), and infrared absorption characteristics (74) are documented. The coordinatively unsaturated species, Ni(CO)2 and Ni(CO)2, also exist and the bonding and geometry data have been subjected to molecular orbital treatments (75,76). 

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

This reaction requires a metal complex that is coordinatively unsaturated and provides another way for bonding of reactant ligands to a metal.Insertion ... 

In this reaction one ligand is inserted between the metal and another ligand, creating a site of coordinative unsaturation so that another reactant ligand can be associated with the metal. The insertion reaction accounts for the chain-growth steps of olefin polymeri2ation reactions. 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

Monomeric sulfur diimides have an extensive coordination chemistry as might be anticipated from the availability of three potential donor sites and two r-bonds. In addition, they are prone to fragmentation to produce thionitroso and, subsequently, sulfido and imido ligands. Under mild conditions with suitable coordinatively unsaturated metal... 

The crucial experiment suggesting that the H2 molecule might act as a dihapto ligand to transition metals was the dramatic observation that toluene solutions of the deep purple coordinatively unsaturated 16-electron complexes [Mo(CO)3(PCy3)2] and [W(CO)3-(PCy3)2l (where Cy = cyclohexyl) react readily and cleanly with Ha (I atm) at low temperatures to precipitate yellow crystals of [M(CO)3H2(PCy3)2] in 85-95% yield. The... 

The reaction may occur with either coordinatively unsaturated or saturated complexes, e.g. ... 

Some of these reactions result, essentially, in the oxidative addition of N0" N03 to coordinatively unsaturated metal centres whereas in others ligand replacement by NO+ occurs — this is a favoured route for producing nitroprusside , i.e. nitrosylpentacyanoferrate(II) ... 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

Recenl work has defined more carefully ihe nature of active sites. Metal surfaces are thought to contain three main types of sites terraces, ledges (or steps) and kinks, which correspond to one, two. and three coordinatively unsaturated sites of organometallic chemistry. These sites display differing activities toward saturation, isomerization, and CKChiingQ 7 J0,68 JO 1.103,104,105). 

The primary photochemical of transition metal carbonyls [Me(CO)n] involves the scission of carbon monoxide (CO) and the formation of coordinated unsaturated species ... 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

RhCl(PPh3)3 is a very active homogenous hydrogenation catalyst, because of its readiness to engage in oxidative addition reactions with molecules like H2, forming Rh—H bonds of moderate strength that can subsequently be broken to allow hydride transfer to the alkene substrate. A further factor is the lability of the bulky triphenylphosphines that creates coordinative unsaturation necessary to bind the substrate molecules [44]. 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

The metallocene dichloride of zirconium and hafnium 20b and 20c were also prepared and underwent reduction with potassium to give monomeric metallocene monochloride complexes 21b and 21c (Eq. 8) [39b]. The structure of the zirconocene complex 21 b in the crystal showed a conformation which suggests a less steric strain as compared to 21a due to zirconium s larger atomic size. As a consequence of the coordinative unsaturation an unusually short Zr —Cl bond length was found. 

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