Platinum complexes molecular orbital calculations

Molecular-orbital calculations performed on platinum antitumor complexes and related species, and force-field calculations carried out on their adducts with nucleic acids are reviewed. The aim of the author is to point out the methodological difficulties encountered in these calculations, and to comment on the (sometimes problematic) results which they have yielded. 

The aim of the present contribution is to critically review computational work related to platinum antitumor drugs, published prior to 1998. After a section devoted to molecular-orbital calculations on platinum antitumor complexes and related compounds, we address force-field calculations on platinum adducts with DNA constituents that have been used (mainly in combination with NMR spectroscopy) to evaluate the structure of the adduct. A brief outlook concludes this chapter. 

Tetrahedral Anions and Related Species. The hydrolysis of the [BHJ anion in moist acetonitrile is an acid-catalysed reaction, and when acetic add is used as a catalyst there appears to be complex formation. The observations are considered to be consistent with BHg as an intermediate, a species which molecular orbital calculations suggest could be metastable. The heat capacity of activation for the hydrolysis of [BHi] has been reported. The decomposition of [BHJ in alkaline solution is catalysed by platinum powder, and a PtH species is suggested as an unstable intermediate. There have been kinetic studies of the alkaline hydrolysis of [BF4]- and [Bp2(OH)2]. A F n.m.r. study of the alkaline hydrolysis of [BF OMe]" is considered to suggest an Nlcb mechanism for the hydrolysis of the first fluoride ion. The acid-catalysed hydrolysis of MeaNBHgNa has been studied in some detail. The kinetics are... 

Chemical considerations suggest that metal-olefin back donation will be less important for silver(I) than for platinum(II), and Basch s ab initio calculations on [Ag(C2H4)]+ (75) have confirmed this view. These calculations suggest that most of the electronic rearrangement of the ethylene unit in this complex ion can be accounted for by the polarization effects induced by the positive charge on the silver atom. Indeed, the bonding metal-olefin molecular orbital has only 6.5% Ag 5s orbital character. This result agrees nicely with recent ESR studies on y-irradiated silver-olefin complexes which estimate a 5s spin density of 4.6% for this molecular orbital 92, 93). 

To understand the physical meaning of the electronic structures of platinum acetylide complexes, theoretical calculations have been employed to deduce information related to the molecular orbitals and the electronic transitions. The first theoretical... 

The extended Hiickel method, which is a semiempirical quantum chemistry method, is often used as a preliminary step in the DFT study of molecular orbital analysis. The acetylide-bridged organometallic dinuclear complexes 5.2 were studied by Halet et al. using the extended Hiickel method for qualitative analysis and DFT for additional electronic properties [97], The extended Hiickel analysis concluded that the main contribution of the Pt-C bond arises from ct type interactions while the n back-donation is very weak. The DFT/BP86 calculation gives a 2.371 eV HOMO-LUMO gap. The electronic communication parameter Hdb between the bis-ferrocene compound linked with platinum acetylide (5.3) was calculated to be 0.022 eV, compared with 0.025 eV obtained experimentally by Rapenne and coworkers using DFT and the extended Hiickel method [98],... 

In order to explain the isomerization of neopentane to isopentane on platinum films, Anderson and Avery [34) proposed a mechanism involving, as precursor, an a,a,y-triadsorbed species, and, in the transition state, a 71 complex of the Dewar type, attached to the surface by two carbon-metal bonds. By simplified Hiickel molecular orbital (MO) calculations, they showed that hyperconjugative effect and partial charge transfer to the metal could account for the relative isomerization rates of the various molecules studied (neopentane > isobutane > n-butane) (Scheme 19). 

In previous sections we have discussed the relativistic effect on the electron density of one-electron atoms and on atomic orbitals, about which many research papers have appeared (see Ref. [1051] for one of the first examples). We shall now consider the case of molecules where polarization effects play a role. The relativistic effect on the electron density of an extended molecule can be conveniently demonstrated by inspection of electron density differences. Figure 16.3 depicts such a density difference for the plane of the atomic nuclei of the mono-acetylene complex Pt(C2H2) [880]. The densities have been calculated from simple Hartree-Fock determinants according to Eq. (8.206) as the sum of squared molecular spinors. The relativistic contraction of the electron density in the core of the platinum atom appears as a huge tower of the dif-... 

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