Dihydrogen complexes stretched

Figure 10. Correlation of the dihydrogen stretching frequency and the dissociation energy Dq for charged dihydrogen complexes.

According to the valence bond order concept as applied to the case of stretched H2 complexes by Limbach, there is a significant H-H bond order phh even at /hh = 1.6 A, implying that stretched dihydrogen complexes are not usefully considered as compressed dihydrides . 

We have already mentioned dihydrogen complexes of Re (e.g. 22.30) and noted the presence of a stretched H—H bond. Other examples include W(CO)3(r -H2)(P Pr3)2 and [OsH(r -H2) P(OEt)3 4]. The H2 molecule only has available a fj-MO (electron donor orbital) and cr -MO (acceptor). Both metal-ligand interactions shown in Figure 23.9 weaken the H—H bond, and coordination readily leads to H—H cleavage (see Section 23.7). 

Upon binding to a metal, the H-H stretching band (v(H-H)) in the IR spectrum red-shifts from the 4300 cm in free hydrogen. A range of 2300 to 3100 cm has been observed for H2 complexes but it is not always seen. The appearance of v(H-D) stretch approximately halfway between the H2 and D2 bands has been taken as convincing evidence of presence of a o-dihydrogen complex. Raman studies on H2 complexes have not been successful because the complexes decomposed under laser irradiation even when exceptional precautions were taken. 

An unusual temperature dependence of /(H,D) has been discovered in several stretched dihydrogen complexes [20,21,25]. To account for the reduced /(H,D) at higher temperature, Klooster et al. [20] proposed the thermal population of the vibrational excited states of the H2 units, where the higher energy states have longer H-H distances. DFT calculation, combined with quantum nuclear motion calculation, has strengthened this hypothesis and predicted an isotope effect on the H-H distance [26]. Very recently, direct experimental evidence for this proposal was reported by Law et al. [27]. 

Table 11.1. N2 and CO Force Constant and Stretching Frequency Data as a Predictive Tool for the Stability of Dihydrogen Complexes (Reference 13)...

The infrared stretching frequency has also been used to distinguish dihydrogen from dihydride complexes. The Raman frequency of free H is 4300 cm and typical v j vibrations are found between 1700 and 2300 cm" . The dihydrogen ligand vibrates at a frequency between that of free H and those of metal hydrides. " Typical values for the H-H stretch in dihydrogen complexes lie between 2400 and 3100 cm" . For example, Kubas s compoimd contained a band at 2690 cm" (see Equation 2.19) for the H-H stretch. Although observed in some cases, the H-H stretch is weak and has not been identified in the majority of H complexes. 

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