Dihydrogen complex
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/bd/H2M.png/300px-H2M.png)
Dihydrogen complexes are
Upon complexation, the H−H bond is extended to 0.81–0.82 Å as indicated by neutron diffraction, about a 10% extension relative to the H−H bond in free H2. Some complexes containing multiple hydrogen ligands, i.e. polyhydrides, also exhibit short H−H contacts. It has been suggested that distances < 1.00 Å indicates significant dihydrogen character, where separations > 1 Å are better described as dihydride complexes (see figure).
Characterization
![](http://upload.wikimedia.org/wikipedia/commons/thumb/8/84/HFe_H2_dppe_2.svg/220px-HFe_H2_dppe_2.svg.png)
The usual method for characterization is 1H
An ideal if nontrivial method of characterization of dihydrogen complexes is
The triangular MH2 subunit has six normal modes of vibration, one of which is mainly of νH−H character. In free H2, this very strong bond absorbs at 4300 cm−1, whereas in dihydrogen complexes the frequency drops to around 2800 cm−1.
Synthesis
Two preparation methods involve the direct reactions with H2 gas. The first entails the addition of H2 to an unsaturated metal center, as originally reported for W(CO)3(P-i-Pr3)2(H2). In such cases, the unsaturated complex in fact features an agostic interaction that is displaced by the H2.
In other cases, H2 will displace anionic ligands, sometimes even halides. Treatment of chlorobis(dppe)iron hydride with sodium tetrafluorborate under an atmosphere of hydrogen is one example:[3]
- HFeCl(dppe)2 + NaBF4 + H2 → [HFe(H2)(dppe)2][BF4] + NaCl
Many metal hydrides can be protonated to give dihydrogen complexes:[4]
- H2Fe(dppe)2 + H+ → [HFe(H2)(dppe)2]+
In such cases, the acid usually is derived from a
History
In 1984, Kubas et al. discovered that the addition of H2 to the purple-colored species M(CO)3(PR3)2 gave a yellow precipitate of mer-trans-M(CO)3(PR3)2(H2) (M = Mo or W; R = cyclohexyl, iso-propyl).)3" described in 1968 was reformulated as a dihydrogen complex.
See also
References
- ISBN 0-306-46465-9.
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