Transition-metal model complexes

A number of papers on some transition-metal model complexes have been reported in the literature to mimic the activity of CA and explore further information on the catalytic mechanism of CA. This aspect has been dealt with in Section IV on formation and aquation/decarboxylation of carbonato complexes. A few more relevant... 

In 2004 and 2005 the photochemical activation of dinitrogen with transition metal model complexes of the Sellmann type nitrogenase was studied using CPMD [193, 194], A dinuclear complex — designed to emulate the open-side FeMoco model -was simulated. Several side reactions were observed which have to be suppressed in order to arrive at the reduced species [194]. Chelate effects and their partial dissociation as well as low temperatures led to successful events. An optimized design of the complexes to inhibit side reactions was suggested [194]. 

To date, most of the photochemical data available for transition metal complexes comes from condensed phase studies (1). Recently, the primary photochemistry of a few model transition metal carbonyl complexes has been investigated in gas phase (5.). Studies to date indicate that there are many differences between the reactivity of organometallic species in gas phase (5.6) as conq>ared with matrix (7-10) or solution (11-17) environments. In most cases studied, photoexcitation of isolated transition metal... 

In conclusion though, it may reasonably be maintained that the ligand field model has contributed significantly to the advances in knowledge concerning transition metal sandwich complexes over the last ten years, and that it offers a valid and convenient framework for further experimental and theoretical progress. 

The metamagnetic behavior of [Fe(Cp )2] [Ni(a-tpdt)2] is attributed to the AF coupling between the FM coupled D+A D+A chains within the chain layers, as predicted from the application of the McConnell I model and spin density calculations, in a similar way to other salts also based on decamethylmetallocenes and other transition-metal bisdichalcogenate complexes with a type I structural motive such as [Mn(Cp )2][M(tdt)2] (M = Ni, Pd, Pt). 

In order to understand the bonding in transition-metal sulfoxide complexes, it is necessary to summarize the physical data available in the literature and so determine what constraints are necessary in any bonding model. An understanding of the factors affecting the bonding in these complexes is essential if further developments are to be made in the chemistry of transition-metal sulfoxide complexes. 

Several workers have pointed out that there is a large and growing body of X-ray structural data of transition metal phosphine complexes. Such data can provide real cone angles, which may then be compared to Tolman s cone angles derived from models. Of course this line only applies to crystalline solids. Since the main application of transition metal chemistry is homogeneous catalysis, crystal structure data, though useful, are of limited applicability. Inevitably, cone angles in complexes in solution will be more variable than in crystalline systems. 

Fritz G, Heizmann CW, Kroneck PM. 1998. Probing the structure of the human Ca2+- and Zn2+-binding protein S100A3 spectroscopic investigations of its transition metal ion complexes, and three-dimensional structural model. Biochim Biophys Acta 1448(2) 264-276. 

In spite of the presence of usually bulky substituents at the silicon atoms, the Si—Si bond lengths are dramatically shortened and approach those of disilenes. This bond shortening is accompanied by a planar or almost planar arrangement of the substituents R1, R2 and the other silicon atom about each silicon atom. Such a geometrical arrangement resembles the bonding situation in transition metal-olefin complexes which, according to the model... 

The most recent view on the 7t-accepting abilities of phosphines in transition metal-phosphine complexes is by Marynick 247) who has used approximate and ab initio MO theory to demonstrate that n accepting into o orbitals is important for PF3. Calculations on the model complex [Cr(NH3)5(PF3)] compared to [Cr(NH3)s(PH3)] showed... 

Nielsen et al. have introduced a monoether linked bis-carbene [209] modelled on an amino linked bis-carbene ligand that acts as a C,N,C pincer ligand in a corresponding palladium(II) complex [156]. Synthesis of the ether linked bis-carbene is facile and involves the reaction of the l-co-dichloro-diethylether with 2 equiv. of methylimidazole. Subsequent reaction with silver oxide and carbene transfer to suitable transition metal precursor complexes affords the corresponding complexes (see Figure 3.73). 

The matrix isolation technique has been applied, in conjunction with the salt/molecule reaction technique, to model the high temperature gas phase reactions of alkali halide salt molecules. The reactions with Lewis acids such as SiFi, HF and CO2 yielded ion pair products which were quenched into inert matrices for spectroscopic study. Difficulties arising from lattice energy considerations in alkali halide salt reactions are minimized by the initial vaporization of the salt reactant. The reaction of such salt molecules with Lewis bases, including H2O and NH3, yielded complexes similar in nature to transition metal coordination complexes, with binding through the alkali metal cation to the base lone pair. 

The matrix isolation experiments using epr, ir, uv-visible and other spectroscopic techniques on transition metal-olefin complexes [8,49] have naturally attracted the attention of theoretical chemists and calculations on the Ni-C2H4 system were reported in one of the first theoretical-experimental papers mentioned in the introduction [16]. These results were later supplemented with a larger (double-zeta) basis set [3Q] and also [31] extended for a Ni(C2H4)2 system. The main conclusions are that a net charge transfer of almost 1/5 of an electron from the metal to the ethylene is evident and that a donation and back donation mechanism consistent with a classical Dewar-Chatt-Duncanson model exists. The Ni-ethylene binding energy is 12.8 kcal/mol. 

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