Density functional theory complexes

Because of the complexity of the pathway, the sensitivity of the reagents involved, the heterogeneous nature of the reaction, and the limitations of modern experimental techniques and instrumentation, it is not surprising that a compelling picture of the mechanism of the Simmons-Smith reaction has yet to emerge. In recent years, the application of computational techniques to the study of the mechanism has become important. Enabling theoretical advances, namely the implementation of density functional theory, have finally made this complex system amenable to calculation. These studies not only provide support for earlier conclusions regarding the reaction mechanism, but they have also opened new mechanistic possibilities to view. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

A currently popular alternative to the ah initio method is density functional theory, in which the energy is expressed in terms of the electron density rather than the wave-function itself. The advantage of this approach is that it is less demanding computationally, requires less computer time, and in some cases—particularly for d-metal complexes—gives better agreement with experimental values than other procedures. 

Allen GC, Warren KD (1974) The Electronic Spectra of the Hexafluoro Complexes of the Second and Third Transition Series. 19 105-165 Alonso JA, Baibas LC (1993) Hardness of Metallic Clusters. 80 229-258 Alonso JA, Baibas LC (1987) Simple Density Functional Theory of the Electronegativity and Other Related Properties of Atoms and Ions. 66 41-78 Andersson LA, Dawson JH (1991) EXAFS Spectroscopy of Heme-Containing Oxygenases and Peroxidases. 74 1-40 Antanaitis BC, see Doi K (1988) 70 1-26... 

Thiourea Ugands can be bounded to the metal centre through one nitrogen atom, the sulfur atom, or the C = S double bond. These coordination modes were studied by density functional theory calculations for Rh-thiourea complexes (Scheme 13). No stable structure was attained by optimisation of the nitrogen coordination mode I but optimised geometries as trigonal-bipyramidal complexes were obtained for modes II and III. An coordination is determined for the latter complex through both S and C atoms. As this... 

A group of investigators recently suggested that the density-functional theory (DFT), which calculates IR and Raman spectra, is a useful tool for direct characterization of the structures of diamondoids with increasing complexity [66]. They applied DFT to calculate Raman spectra whose frequencies and relative intensities were shown to be in excellent agreement with the experimental Raman spectra for C26H30, thus providing direct vibrational proof of its existence. 

The authors carried out theoretical calculations on this system as well as on the [ (PMej) ] system to compare their reactivity with hexafluorobenzene. They found that the barrier for [ (liPr) ] is approximately 10 kJ/mol lower in energy than the corresponding barrier for the phosphine-bearing system. This value was in agreement with the different reactivity of both complexes but could not fully explain the large difference in reaction times. Density functional Theory (DFT) calculations also showed that the trans product is more stable than the cis product (total energies are respectively -130.9 and 91.1 kJ/mol), which was in agreement with the experimental values. 

The exact nature of the catalytically active Ni species in these reactions is yet to be conclusively established. Hydrodechlorination proves optimal with a NHC Ni ratio of 2 1 suggesting that 14-electron Ni(NHC)2 is involved, whereas the 1 1 NHC Ni ratio necessary for hydrodefluorination implies that it is the 12-electron mono-carbene adduct Ni(NHC) which is catalytically active [10]. Smdies by Matsubara et al. revealed that treatment of NKacac) with either one or two equivalents of IMes HCl 1 or SlMes HCl 2 in the presence of NaOHu formed the mono-NHC complex Ni(NHC)(acac)j which, upon reduction with NaH in the presence or absence of carbene, formed Ni(NHC)2 [11]. Density functional theory (DFT) calculations suggest that the strength of the Ni-NHC bond (ca. 50 kcal/mol) makes... 

Another stndy on binding to NHC complexes, that combined experiments and DFT (density functional theory) calculations was recently reported on a ruthenium system. This study shows the reversible binding of oxygen to the tetra-NHC complex [Ru(NHC) H)][BAr/] 6 (BAr/ = B (3,5-CF3) C H3 ), which leads to complex 7 (Scheme 10.2) [12]. Unexpectedly, the chemical shift of the hydride ligand undergoes a large downfield shift upon coordination to (from -41.2 ppm... 

Density functional theory (D FT) modeling calculations show that a dinuclear gold(I) amidinate complex is less stable than the tetranuclear gold(I) amidinate cluster, [Au4(HNC(H)NH)4]. However, replacing C by Si in the backbone reduces ring strain and makes the energies similar. Figures 1.21 and 1.22 [39]. 

Figure 1.22 Density Functional Theory calculations of the tetranuclear and dinuclear amidinate complexes at both the Gaussian 98 and ADF levels.

Hertwig, R.H., Hrusak, J., Schroder, D., Koch, W. and Schwarz, H. (1995) The metal-ligand bond strengths in cationic gold(l) complexes. Application of approximate density functional theory. Chemical Physics Letters, 236, 194-200. 

Lev, D. A. Grotjahn, D. B. Amouri, H. Reversal of reactivity in diene-complexed o-quinone methide complexes insights and explanations from ab initio density functional theory calculations. Organometallics 2005, 24, 4232 -240. 

During the last years, more and more researchers have applied density functional theory to small transition-metal complexes and benchmarked the results against either high level wave function based methods or experimental data. A particular set of systems for which reasonably accurate benchmark data are available are the cationic M+-X complexes, where X is H, CH3 or CH2. Let us start our discussion with the cationic hydrides of the 3d transition-metals. 

In reviewing the performance of density functional theory applied to hydrogen bonded complexes of moderate strength, we repeatedly noted a systematic underestimation of the interaction energies for many types of functionals, usually below 2 kcal/mol. This has been related by some researchers to the inability of modem functionals to describe those contributions to intermolecular binding energies which stem from dispersion forces. Dispersion... 

Bray, M. R., Deeth, R. J., Paget, V. J., Sheen, P. D., 1996, The Relative Performance of the Local Density Approximation and Gradient Corrected Density Functional Theory for Computing Metal-Ligand Distances in Werner-Type and Organometallic Complexes , Int. J. Quant. Chem., 61, 85. 

Deng, L., Ziegler, T., 1997, Theoretical Study of the Oxidation of Alcohols to Aldehyde by d° Transition-Metal-Oxo Complexes Combined Approach Based on Density Functional Theory and die Intrinsic Reaction Coordinate Method , Organometallics, 16, 716. 

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