Hydride complexes polarization

A Fe-H bond is generally polarized as Fe -H because H is more electronegative than Fe. However, iron hydride complexes impart much less negative charge to the hydride than early transition-metal hydride complexes. 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

Clapham, S.E., Hadzovic, A. and Morris, R.H. Review Mechanisms of the H2-Hydro-genation and Transfer Hydrogenation of Polar Bonds Catalyzed by Ruthenium Hydride Complexes. Coord. Chem. Rev., 2004, 248, 2201-2237. 

A major question is whether heterolytic cleavage of the H3C-H bond occurs as depicted in Scheme 10 (maintaining the Ptn state) or whether oxidative addition to a PtIV methyl hydride complex takes place. In such systems transfer of protons would be expected to be very facile because of the extremely high mobility of H +, and even a short-lived, very weak a complex could be a key intermediate. The C-H bond is likely to be polarized towards Cs H5+ on such highly electrophilic cationic metal complexes, where H+ can very rapidly split off and transfer to either a cis ligand or the anion as soon as the... 

Alkenes and metal hydrides experimental studies 66 Alkenes and metal hydrides theoretical studies 67 Elimination of metal hydrides from organometallics 68 Complex metal hydrides and polar functional groups experimental studies 69... 

Complex metal hydrides and polar functional groups theoretical studies 72... 

COMPLEX METAL HYDRIDES AND POLAR FUNCTIONAL GROUPS EXPERIMENTAL STUDIES... 

Polar apiotic solvents, such as HMPA, and long reaction times were effective. The reaction of rhodium hydride complexes with CO3 in the presence of II3O was found to form the dihydrido bicarbonato complexes RhH3(OjCOH)L2 which reduce CO to form the corresponding Rh(l)[Pg.203]

The dual function of the precatalysts 4 opened the way to well-controlled block polymerization of ethylene and MMA (eq. (5)) [89, 90]. Homopolymerization of ethylene (Mn = 10000) and subsequent copolymerization with MAA (Mn 20000) yielded the desired linear AB block copolymers. Mono and bis(alkyl/silyl)-substituted flyover metallocene hydride complexes of type 8 gave the first well-controlled block copoymerization of higher a-olefins with polar monomers such as MMA or CL [91]. In contast to the rapid formation of polyethylene [92], the polymerization of 1-pentene and 1-hexene proceeded rather slowly. For example, AB block copolymers featuring poly( 1-pentene) blocks (M 14000, PDI = 1.41) and polar PMMA blocks (M 34000, PDI = 1.77) were obtained. Due to the bis-initiating action of samarocene(II) complexes (Scheme 4), type 13-15 precatalysts are capable of producing ABA block copolymers of type poly(MMA-co-ethylene-co-MMA), poly(CL-co-ethylene-co-CL), and poly(DTC-co-ethylene-co-DTC DTC = 2,2-dimethyltrimethylene carbonate) [90]. 

Although this has been a topic of some controversy, it is now widely accepted that electronically saturated hydride complexes have a bond polarity,... 

The importance of lithium hydride in the production of simple and complex hydrides is dependent upon certain unique properties. For example, according to Hurd (19 )y it is the very slight solubility of lithium hydride in polar organic compounds as well as the ability to sustain metathetical reactions that provides for the production of numerous hydrides. This is true for lithium hydride, whereas the other alkali and alkaline earth metal hydrides are insoluble in polar organic solvents, and their metathetical reactions do not proceed at all or at a slow rate at best (19,29). Greater use of lithium hydride is possible in such reactions because of its low dissociation pressure at its melting point of 680 °C. 

There seems to be some confusion about the polarization of the metal-hydrogen bond in certain hydrido-transition metal-carbonyl compounds, in particular HCo(CO)4. This and some related hydrido-carbonyl compounds are slightly soluble in water, and such solutions are clearly acidic In the gas phase or in non-polar solvents, however, these complexes exhibit spectroscopic and chemical properties similar to those of other non-acidic hydride complexes. A MO calculation of the charge densities in HCo(CO)4 indicated that 1.6 electrons are associated with the hydrogen atom, and the question was discussed as to how a molecule with a negative-... 

Clapham SE, Hadzovic A, Morris RH (2004) Mechanisms of the H2-hydrogenation and transfer hydrogenation of polar bonds catalyzed by ruthenium hydride complexes. Coord Chem Rev 248 2201-2237... 

In less polar solvents it is impractical to determine pK values, either because the solvent is not readily protonated (CH Cy or because the solvent permits extensive ion pair formation and its protonated form is unstable (THF). However, the positions of proton transfer equilibria in such solvents (e.g.. Equation 3.125) have been used to estimate relative aqueous pfC/s for hydride complexes that are insoluble in (or unstable in) water. CyjPH, whose aqueous pfC is 9.7, has often been used as an anchor—that is, the acidities of other RjPH and hydride complexes have been measured relative to CyjPH, leading to the "pseudo-aqueous" pJacidity scales in CHjClj and in THF. However, complications due to ion pairing make such pK values less reliable than direct measurements in CHjCN (or water). Morris has linked his THF acidity scale with acidities in DMSO, a solvent that minimizes ion pair formation, but is incompatible with many organometallic complexes. ... 

In view of the acidity of hydride complexes, one might expect them to form hydrogen-bonded intennediates during proton-transfer reactions. However, it is relatively uncommon for neutral hydride complexes to form hydrogen bonds M-H A because most M-H bonds are polarized as M(8+)-H(8-), not as M(8-)-H(8+). (The need to reverse the observed polarization during deprotonation is a major cause of the low kinetic acidity of transition metal hydrides, mentioned previously.) The first M-H Ahydrogen bond from a neutral hydride has just been reported CpM(CO)3H (M = Mo and W) serves as a hydrogen bond donor to (octyl)3P=0 and even to pyridine, apparently because there is M(8-)-H(8+) polarization in its M-H bond. °... 

It is more common for cationic hydride complexes to serve as hydrogen bond donors, presumably because the positive charge polarizes the M-H bond away from the hydride ligand. Examples are (Cp )psH " and WH3(dppe)j in Figure 3.40. ° " ... 

The formation of the M-H +- Y hydrogen bond involves the transfer of electron density from the HOMO orbital of the base to an empty cr orbital of the hydride complex as in the case of any classical hydrogen bond. The induced change of the M-H bond polarization (Apol(M-H), Table 8.2) is similar to that observed for C-H- -Y bonds of halomethanes with nitrogen bases [26] but occurs at notably lower energies of ny to donation [16]. This easier polarization facilitates... 

Eucken et al. [15] showed that dehydration occurs through activated hydride complex, where the ions are polarized, according to the scheme ... 

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