Terminal hydrogen ligands

Hydrido complexes structural units formed by central metal atoms and terminal hydrogen ligands... 

Quantum mechanical exchange coupling (QMC) is a rare but interesting property of some polyhydrides in which different terminal hydrogen ligands need to be present in different chemical environments but must not exchange fast on the NMR timescale. For example, in [CpIr(L)(H)3]+ the three H s lie in a plane with two flanking Ha sites and one Hb in the middle position and with a 4hh of ca. 1.68 A. 

These complexes have one T metal atom at their center and have terminal hydrogen ligands only. Those currently known are summarized in Table 6.2. 

Hydrogen is found in transition metal cluster directly attached to metal atoms, in some few cases as terminal ligand but more frequently bridging two, three, or more metal atoms. Examples of the uncommon terminal hydrogen ligands are found in the complex H2 0s3(C0)u illustrated in Fig. 3.3. 

The bidentate oxazoline ligands 85 and 86 (and derivatives thereof) are excellent reporter ligands, and several studies have used NOEs to determine the nature of their chiral pockets [61, 113, 114, 126]. NOESY studies on the cations [Ir(l,5-COD)(86)]+ and several cationic tri-nudear Ir(iii)(hydrido) compounds [110], e. g. [Ir3(p3-H)(H)5(86)3] +, 87, in connection with their hydrogenation activity, allowed their 3-D solution structures to be determined. In addition to the ortho P-phenyl protons, the protons of the oxazoline alkyl group are helpful in assigning the 3-D structure of both the catalyst precursors and the inactive tri-nudear dusters. Specifically, for one of these tri-nudear Ir(iii) complexes, 87 [110], with terminal hydride ligands at d -17.84 and d -21.32 (and a triply bridging hydride at 5 -7.07), the P-phenyl and oxazoline reporters define their relative positions, as shown in Scheme 1.5. 

Zirconium(IV) and hafnium(IV) tetrakis(tetrahydroborates) M(BH4)4 are of interest as extremely volatile, covalent complexes that contain tridentate BH ligands and exhibit rapid intramolecular exchange of bridging and terminal hydrogen atoms. 4,615 These compounds were prepared initially from NaMF5 (M = Zr or Hf) and excess A1(BH4)3 (equation 53),616 but they are obtained more conveniently from the reaction of the anhydrous metal tetrachloride with excess lithium tetrahydroborate (equation 54), either in the solid state617,618 or in the presence of a small amount of diethyl ether.619... 

TaeClistXCN).,]4- consists of a regular Ta6 octahedron with terminal cyanide ligands. The Ta—C and C—N distances are comparable to those found in normal cyanide complexes. Each nitrogen atom is hydrogen-bonded to two oxygen atoms. 

Fig. 3. Intramolecular hydrogen bonds involving two terminally coordinated ligands (a type) as shown in A (X = OH2, NH3, or OH ) or a terminally coordinated ligand and bridging hydroxide (/I type) as shown in B (X = OH2 or NH3) or in C.

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