Transition metal complexes mass spectroscopy

Included for the first time in this section are books and surveys on spectroscopic techniques which are of great use to the organometallic chemist. In addition to the Chemical Society Specialist Periodical Reports on Spectroscopic Properties of Inorganic and Organometallic Compounds (B6.1) and Mass Spectrometry (B6.2), a series of Annual Reviews (volumes 1 or 2) or Reports (volume 3 onwards) on Nuclear Magnetic Resonance Spectroscopy contains surveys of or P NMR results appertaining to organo-transition metal complexes. 

Molecules that contain heavy elements (in particular 5d transition metals) play an important role in the photochemistry and photophysics of coordination compounds for their luminescent properties as well as for their implication in catalysis and energy/electron transfer processes. Whereas molecular properties and electronic spectroscopy of light molecules can be studied in a non-relativistic quantum chemical model, one has to consider the theory of relativity when dealing with elements that belong to the lower region of the periodic table. As far as transition metal complexes are concerned one has to distinguish between different manifestations of relativity. Important but not directly observable manifestations of relativity are the mass velocity correction and the Darwin correction. These terms lead to the so-called relativistic contraction of the s- and p- shells and to the relativistic expansion of the d- and f- shells. A chemical consequence of this is for instance a destabilisation of the 5d shells with respect to the 3d shells in transition metals. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

Duncan, M.A. (2008) Structures, energetics and spectroscopy of gas phase transition metal ion/benzene complexes. Int. J. Mass Spectrom. Ion Processes, 272, 99-118. 

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