Elongator complex destabilization

Microwave experiments also show that OsH2(CO)4 is a dihydride, and the PES for L HOsH is essentially harmonic without the flat region, while the Ru species shows intermediate behavior.104 However, the reaction profile for A of H2 to Os(CO)4 does show a midpoint plateau corresponding to formation of an elongated H2 complex.20 The nature of the electronic state of the complex may play a large role, as will be discussed below for H2 addition to Fe atoms. M(CO)4(H2) is destabilized by repulsion between a filled d orbital and the filled ff(H2) orbital and by poor BD from the low-energy d orbitals.20... 

The observed tetragonal distortion of an octahedral complex is accompanied by a change in symmetry (Oh to Z)4h) and a splitting of the Cg and t2g sets of orbitals (see Figure 21.10). Elongation of the complex (21.5) is accompanied by the stabilization of each d orbital that has a z component, while the dxy and d 2 orbitals are destabilized. 

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