H-B Systems

An excellent comprehensive review of the chemistry and structures of metal-BH4 complexes has been published by Marks and Kolb85). The attachment of a BH4 ligand to a metal atom can assume either unidentate (III), bidentate (IV) or tri-dentate (V) forms, which are often distinguishable from each other on the basis of M—B distances86-89). 

In addition, for metal complexes of the larger boron hydrides, more complicated types of bonding (where M-H-B linkages are augmented by M-B, M-M or B-B bonds) are found. As mentioned earlier in this article, metalloborane complexes are generally unsuitable for neutron diffraction because of the very large absorption cross 

The tridentate mode of attachment is also rather common. The first structural report of this arrangement appeared in 1967, when the X-ray analysis of Zr(BH4)4 (Fig. 19) was described by Bird and Churchill99). However, the twelve bridging H atoms could not be located in that study. The correctness of the tridentate-bridged model was later confirmed by two independent gas-phase electron diffraction investigations 10°), which came up with measurements of Zr-H = 2.21 (4) A, B—H = 

The analogous molecule Hf(BH4)4 has been analyzed twice by neutron diffraction  

From the limited amount of accurate data available, one can say that M—H—B angles generally decrease in the order M(/i-H)BH3, M(ju-H)2BH2, M(ju-H)3BH e.g., M-H-B = 121.7(4)° in Cu(PPh2Me)3(BH4), 96.8(5)° in Hf(CsH4Me)2(BH4)2, 80.6(6)° in Hf(BH4)4. And it can be shown that M—B distances also decrease in the above sequence, provided that differences in covalent radii are taken into account. 

---

Generate all initial conformer states having at least one A-H...B system in which the H...B distance is in the 2.5-5.0A range. 

An obvious questions which arises is what factors are responsible for E(HB) A general A-H...B system is shown in Figure 6. The energy E(HB) is proportional as. 

However, in the (A.. H.. B) system (more particularly in those represented by (A—H—A) and (B—H—B)+) containing strong hydrogen bonds (Table 1), other types of potential curves may exist. This is the case of the hesitating proton characterized by a small or zero potential barrier. We can distinguish, 1. the asymmetric double minimum, 2. the symmetric double minimum, and 3. the symmetric single minimum by spectroscopic means (see Section III). 

Assume that the A H- B system is in the ground electronic state and that the Boin-Oppenheimer approximation is valid so that the system motion reduces to (Va H- Nb atoms moving on a single potential energy suiface. The phase space of the A-l-B system, excluding overall translation, has 6(A ab — 1)... 

---