Highest occupied molecular orbital transition-metal complexes

Figure 2 Simplified MO schemes showing the effect of jt-interaction between the rhenium atom and the halide axial ligand on the character of the lowest electronic transition of [Re(CO)3(N-N)X]. The two highest-occupied molecular orbitals of the chloro (a) and iodo (b) complexes were predominantly of metal dTr(Re) and iodo pji character, respectively, giving rise to essentially (d 7r(Re) — jr (N-N)) MLCT and (p(I) — 7r (N-N)) LLCT lowest transitions, respectively. (Reprinted fi om Ref. 15, 1998, with permission from Elsevier)...

It is important to note for the following discussion that in electron-transfer processes the reductant s highest occupied molecular orbital (HOMO) should combine with the oxidant s lowest unoccupied molecular orbital (LUMO) of the same symmetry to ensure proper overlap of reductant and oxidant orbitals to initiate electron transfer. That is, electron transfer will occur readily from n to n orbitals on different species or from a to a but not n to a in a linear arrangement of atoms [e.g., A-B-C in Appendix I (following references at the end of this chapter)]. In the case of outer-sphere electron-transfer processes, n- to 7r-electron transfers are favored over a to a because (1) such transfers do not require major changes in bond lengths in the precursor complex (lower activation energy) and (2) the n orbitals are more diffuse or better exposed than a orbitals. This process is well documented for transition metals. For inner-sphere electron-transfer processes, both n- to n- and a- to n-electron transfers are most favored (Purcell and Kotz, 1980). 

We are only interested in the localized wave function of the electron, for example, the highest occupied molecular orbital (HOMO), of the transition state complex with the electron transferred. Hence, we may use the HOMO wave function of the transition state complex after the electron transfer, including only the nearby surface metal atoms that contribute significantly to this HOMO [40]. This wave function at the cluster is calculated using the EHMO method together with the parameters of VSIP and double-zeta orbitals given in [31] ... 

Acrylonitrile appears to coordinate to transition metal ions primarily through its carbon-carbon double bond (highest occupied molecular orbital of that molecule96 ). This mode of coordination has been deduced from the absence of the stretching frequency nc=c97 and, in the cases of Fe(CO)4(CH2=C-CN)98 and Ni(PR.3)2(CH2=CH—CN)99, confirmed by X-ray structural analysis. The CN bond of the acrylonitrile ligand is slightly weakened in these complexes, as shown in a small increase of the bond length and decrease of t>cN-... 

F. Fenske. We demonstrate for transition metal complexes that the non-empirical Fenske-Hall (FH) approach provides qualitative results that are quite similar to the more rigorous treatment given by density functional theory (DFT) and are quite different from Hartree-Fock-Roothaan (HFR) calculations which have no electron correlation. For example, the highest occupied molecular orbital of ferrocene is metal based for both DFT and FH while it is ligand (cyclopentadienyl) based for HFR. In the doublet (S = 1/2) cluster, Cp2Ni2(pi-S)2(MnCO)3, the unpaired electron is delocalized over the complex in agreement with the DFT and FH results, but localized on Mn in the HFR calculation. A brief description of the theory of FH calculations is used to rationalize the origin of its similarity to DFT. 

To illustrate the tuning aspects of the MLCT transitions in ruthenium polypyridyl complexes, the well known [RuLs] (L = 4,4 dicarboxylic acid-2,2 -bi-pyridine) type of complex can be considered. This complex shows strong visible band at 466 nm, because of CT transition from metal t2g highest occupied molecular orbitals (HOMO) to jr -lowest unoccupied molecular orbitals (LUMO) of the ligand (Fig. 3). The Ru(II)/(III) oxidation potential is at 1.3 V, and the ligand based reduction potential is at —1.5V... 

Compared to NHCs, the CAACs have considerably different electronic and steric properties, as evidenced by their ability to activate small molecules (such as dihydrogen), stabilize radical species, and promote the formation of unusually low-coordinate transition metal complexes [5]. Replacing the o-electron-withdrawing amino group in a standard NHC with a o-donating carbon atom serves to raise the energy of the HOMO (highest-occupied molecular orbital)... 

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