Elongated 2-H2 Complexes

Although the rfee ranges shown are arbitrary, each category of complexes has distinct properties. The dun is relatively short (0.8-1.0 A) in trae H2 complexes best exemplified by W(CO)3(PR3)2(H2), much as in physisorbed H2 where dun is <0.8 A. Importantly the H2 binding is often completely reversible here, i.e. H2 can be removed simply upon exposure to vacuum and re-added many times at room temperature. Elongated H2 complexes and compressed... 

In elongated H2 complexes, experimental and computational studies indicate that the 7 -H2 ligand is greatly delocalized and caimot be envisaged as a fixed, rigid unit [7]. Rapid motion of two hydrogen atoms occurs on a flat potential energy surface with a shallow minimum at the neutron-diffraction determined position of 1.2 A for trans-[OsCl(H2)(dppe)2]+. 

Highly Mixed H-H and M-H2 Modes in Elongated H2 Complexes New Normal Mode Definitions... 

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 shape of the PES along the dm coordinate in elongated H2 complexes is also of crucial importance in understanding the structure and bonding in elongated complexes. If these complexes are considered to be species where scission of H2 is arrested at an intermediate stage between the and MH2 structures, the... 

Figure 4.17. Qualitative representation of the potential energy of H-H stretching in hypothetical structures for an Os-H2 complex (I, left-side curves) and a cis-dihydride (II, right-side curves). A and correspond to small electronic coupling where both tautomers may exist, and C represents strong coupling leading to an elongated H2 complex. Reprinted with permission from Craw et aL91 Copyright 1994 American Chemical Society.

Nuclear motion quantum calculations (DVR methodology see Section 8.4) give somewhat lower EIE values (0.53 at 300 K), but unexpectedly show normal EIE for elongated H2 complexes such as... 

True H2 complex Elongated H2 complex Compressed dihydride Dihydride... 

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