Chemical shifts transition metal complexes

Fourier-transform infrared (FTIR) spectroscopy Spectroscopy based on excitation of vibrational modes of chemical bonds in a molecule. The energy of the infrared radiation absorbed is expressed in inverse centimeters (cm ), which represents a frequency unit. For transition-metal complexes, the ligands -C N and -C=0 have characteristic absorption bands at unusually high frequencies, so that they are easily distinguished from other bonds. The position of these bonds depends on the distribution of electron density between the metal and the ligand an increase of charge density at the metal results in a shift of the bands to lower frequencies. 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

It is evident from the above table that a considerable spread of chemical shift values is observed in tellurium-transition metal complexes, but the factors that determine the chemical shift are still poorly understood and data are not available for all known structural types. The most extensive compilations of data have been provided by Rauchfuss (187) and Herrmann (191), with the point being made in the former reference that chemical shifts are extremely sensitive to changes in cluster geometry. In principle, 12sTe NMR spectroscopy is a valuable method for studying tellurium-transition metal clusters in solution, but it is clear that more data are required before unambiguous structural assignments can be inferred. 

While DFT may or may not be more accurate than MP2 for absolute shielding calculations is debatable, the strength of the DFT method in calculations of shieldings is in the ability of DFT to provide a consistent picture over a wide range of chemical systems, since calculations can be done at a very modest computational cost compared to MP2. Among the successes of the method is in ligand chemical shifts in transition metal complexes. For example, 13C, 170,31P and H chemical shifts for oxo (12,14,15), carbonyl (16-19), interstitial carbide (20), phosphine (21,22), hydride (23), and other ligands have been successfully reproduced to within tens of ppm in... 

In the last few years, DFT has also become one of the prime methods for the study of nuclear magnetic resonance (NMR) chemical shifts in transition metal complexes and other large molecules. DFT calculations of NMR chemical shifts have been reviewed (3,4). 

With the advent of appropriate DFT-based methods, NMR properties of transition-metal compounds have now become amenable to theoretical computations (8). Suitable density functionals have been identified (9) which permit calculations of transition-metal chemical shifts with reasonable accuracy, typically within a few percent of the respective shift ranges. Thus, it is now possible to investigate possible NMR/reactivity correlations for transition-metal complexes from first principles several such studies have already been undertaken (10,11,12). 

The rate of elementary reactions of certain transition-metal complexes, such as insertions or substitutions, can be controlled by the substituents at the metal center. In favorable cases, usually in families of closely related systems, these substituents can affect the reactivities and the chemical shifts of the transition metal nuclei in a similar, parallel fashion, resulting in an apparent correlation of both properties. Modem DFT methods can reproduce these findings, provided that changes in rate constants are reflected in corresponding trends in activation barriers or BDEs on the potential energy surface. 

Keywords Density functional calculations Transition metal complexes NMR chemical shifts Nuclear spin-spin coupling constants Relativistic quantum chemistry... 

Early work on the kinetics of photoinduced ET in transition metal complex systems focused exclusively on bimolecular reactions between transition metal chromophores and electron donors or acceptors. However, concomitant with the advances in rapid photochemical kinetic methods and chemical synthetic methodology, emphasis shifted to photoinduced ET in chromophore-quencher assemblies that comprise a metal complex chromophore covalently linked to an organic electron donor or acceptor [24]. These supramolecular compounds afford several... 

For those familiar with NMR spectroscopy it may be helpful to realize that the ESR g-shift is comparable with the NMR chemical shift. Similarly, electron-nuclear hyperfine coupling can be compared with nuclear-nuclear spin-spin coupling in NMR. (In systems containing more than one unpaired electron per molecule, electron spin-electron spin coupling is, of course, important. For doublet-state radicals, this coupling does not arise it is of great importance in triplet state molecules and in many high-spin transition metal complexes.)... 

TABLE XVIII The Chemical Shifts of Some Paramagnetic Transition Metal Complexes ... 

The redox potentials for the electron acceptors that react with HO (Table 15) are such that a pure outer-sphere single-electron transfer (SET) step would be endergonic (the HO /HO redox potential is more positive than the redox potential of the electron acceptor). Hence, the observed net reactions must be driven by coupled chemical reactions, particularly bond formation by the HO to the electrophilic atom of the acceptor molecule that accompanies a singleelectron shift. (The formation of the bond provides a driving force sufficient to make the overall reaction thermoneutral or exergonic 1.0 V per 23.1 kcalmol of bond energy.) The effect of various transition metal complexes on the oxidation potential for HO in MeCN illustrates some of these effects the results are summarized in Table 16. ... 

Displacement of a good leaving group from silicon in a noncoordinating solvent (equation 5) allows solvent-free complexes (1) to be prepared when the substituents on silicon are bound via sulfur. As would be expected by analogy with carbene complexes, the Si NMR chemical shift for these complexes is at very low held (264.4 and 268.7 ppm, respectively, for R = S-/ -tolyl and SEt, respectively) and they also react readily with donor molecules such as MeCN to give four-coordinate, donor stabilized silylene species. (See Section 6 below for further details of transition metal silyl complexes.)... 

Because of solvent differences, precise comparison of the chemical shift values of alkali metal 1,2-heteroborolides with those of their transition metal complexes 126, 117, and 114 is tenuous. However, in general, the spectra of the transition metal complexes show an upfield shift consistent with 7t-coordination . The values of of the transition metal complexes are similar to those of their alkali metal salts. 

In general, B NMR chemical shift values are a sensitive function of electronic effects about the boron atoms . The chemical shift values of the neutral unsaturated heterocyles 120-122 occur in the range of S 39-44. On deprotonation to form 123-125, there is an upfield shift. This shift is consistent with an increase in the electron density at boron which suggests an enhanced 7t-bonding to the adjacent atoms. On coordination to form transition metal complexes 126, and 114, there is a further upfield shift indicative of direct metal rt-bondingto boron. 

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