Cp* ligands

In 1987 Tapper et al. reported the first stable Mo and W complexed phosphinidenes 52 with two Cp ligands on the transition metal from the reaction of Mes PCl with Cp2M(H)Li [91]. Niecke et al. approached the syntheses of similar compounds with instead a NHMes substituent using Mes NPCl and Cp2M(H)Li... 

The covalent part of the bonding character of both Cp ligands with Ee(ll) can be summarized by c-bond interactions (Cp slpjd22), strong 7i-interactions (Cp d jdyjpjpy), and weak retrodative 8-interactions (d y/d 2-y2 Cp )... 

As a consequence of the molecular orbital interactions, ferrocene adopts an axially symmetrical sandwich structure with two parallel Cp ligands with a distance of 3.32 A (eclipsed conformation) and ten identical Fe-C distances of 2.06 A as well as ten identical C-C distances of 1.43 A [12]. Deviation of the parallel Cp arrangement results in a loss of binding energy owing to a less efficient orbital overlap [8]. All ten C-H bonds are slightly tilted toward the Fe center, as judged from neutron-diffraction studies [13]. 

Along these lines, imidazoline [68, 69] rather than oxazoline coordination sites better fitted into a modular concept in order to understand how the electronic properties should ideally be, since electron density can be readily adjusted by the choice of the N substituent [70]. The Cp ligand C5R5 in 47 permits steric and electronic tuning. With the electron-withdrawing effect of five Ph groups R, the... 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

Very recently another highly active and well-defined Pd-NHC based pre-catalyst containing a cyclopentadienyl (Cp) ligand 18 has been successfully applied in this transformation. Cp was chosen as stabilising ligand due to its well-known tendency to reductively be removed from Cp-Pd complexes that may help in the transformation of the pre-catalyst into the desired catalytic active species (NHC)Pd(O) [107]. Di- and tii-ortho substituted biaryls were obtained in good to excellent yields however, when the formation of tetra-orf/to substituted compounds was attempted very poor yields were obtained, even using aryl bromide or iodide substrates (Scheme 6.28). 

Very recently, the yttrium hydride [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -di-methylbiphenyl]YH(THF) 2 (36), conveniently generated in situ from [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -dimethylbiphenyl]YMe(THF)2 (35) demonstrated its high catalytic activity in olefin hydrosilylation. This system represents the first use of a d° metal complex with non-Cp ligands for the catalytic hydrosilylation of olefins. Hydrosilylation of norbornene with PhSiHs gave the corresponding product (37) of 90% ee (Scheme 3-15) [43]. 

The hydrido(ethoxo) complex carrying an electron-donating q -CsMes (= Cp ) ligand, [Cp IrH(OEt)(PPh3)] (4), was prepared by a metathesis reaction between [Cp Ir Cl2(PR3)] (3) and NaOEt followed by P-H elimination from the intermediate diethox-ide complex (Eq. 6.4) [7]. Several other iridium alkoxide analogs [Cp IrH(OR)... 

Substitution of the Cp ligands reduces the tendency to dimerize. Introduction of a cyclohexyl group is sufficient for rendering the monomer the only detectable species by CV. The substituted titanocene chlorides open epoxides slower than Cp2TiCl . However, the resulting /J-metaloxy radicals are more... 

Instead, transannular strain between the silylether and one of the Cp ligands leads to a product 77 from a boat-like insertion transition structure (76 bottom). 

A change in hapticity of the Cp ligand in 1 from q3/5 to ri1 should lead to a drastic change in the character of the molecule whereas the 7t-bonded species (I) can be regarded as a hyper-coordinated, electronically over-saturated and thus nucleophilic silylene, the a-bonded species (II) represents an example of an electron deficient and thus electrophilic silylene. 

Hydride addition to [CoCp(CsH5BPh)]+ (1) occurs at C-2 and C-4 as well as at the Cp ligand. The products 67 with an (r/5-l-bora-2,4-... 

Organocobalt complexes catalyze the cyelocotrimerization of acetylenes and nitriles, which affords pyridine and benzene derivatives (100). (Cyclo-pentadienyl)cobalt complexes such as CoCp(COD) favor pyridine formation (100), and modification of the Cp ligand has considerable influence on the activity of the catalyst and the chemo- and regioselectivity of the catalytic process (101). 

Fragalia and co-workers have reported the details of the He(I) and He(II) excited photoelectron spectra of Cp2Ti(CO)2 and concluded that evidence exists for significant backbonding between the Ti 3d orbitals and empty carbonyl v orbitals. Further, there is no evidence of important overlap between Ti and Cp orbitals. A small electrostatic perturbation of the Cp ligands is caused by the titanium atom (85). Bohm has described an elaborate study of the low energy PE spectrum of Cp2Ti(CO)2 (1) by means of semiempirical MO calculations (86). 

Studies of CpLi solutions showed high conductivity, suggesting ionic particles in solution.91 Ionic particles could consist of the free ions, but anionic cyclopentadienides MCp2- particles isoelectronic to the well-known alkaline earth cyclopentadienides Cp2M (M = alkaline earth metal),103 could also be envisioned. Moreover, the structural motif of a sandwich complex where two Cp ligands bind to a metal center is well established throughout the periodic table. Indeed, the Cp2 Li anion was suggested based on NMR techniques,114-116 and... 

In a search for CVD precursors for beryllium, the air sensitive, volatile solid CpBe(SiMe3) 17 was prepared from the reaction of Li[SiMe3] with CpBeCl in pentane.62 Characterized with single crystal X-ray diffraction, multinuclear NMR, and mass spectra, the compound displays a Be-Si bond length of 2.185(2) A that is somewhat longer than the sum of covalent radii (2.01 A). The lengthening is similar to that observed in the related Be(Si(/-Bu)3)2 (Be-Si = 2.193(1) A),63 so there is not a pronounced effect from the Cp ligand on the Be-Si interaction. 

The first complex exhibiting 7r-coordination of a Cp ligand to a Cd(ll) center in the solid state, namely, [Cp2Cd-tmeda] 196, has been recently isolated and structurally characterized (Figure 33).254 Both Cp rings interact with the cadmium center in an -fashion with Cd-C distances ranging from 2.34 to 2.74A. 

By using conformationally variable Cp ligands such as C5H4Pr1, change in the dominant mode of propylene polymerization from isotactic to syndiotactic was accomplished by varying the reaction temperature [172]. 

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