HD complex

If deuterio acids are used then ij -HD complexes are formed these are particularly useful in establishing the retention of substantive H-H bonding in the coordinated ligand by observation of a 1 1 1 triplet in the proton nmr spectrum (the proton signal being split by coupling to deuterium with nuclear spin 7 = 1). 

An attempted synthesis of the deuterated analogue of Complex 7 using a D2 instead of a H2 atmosphere leads to the expected di-deuteride, Complex 8, together with appreciable amounts of the dihydride and HD complex, 9. The IR p(lr-H and Ir-D) clearly reveals a hydride and a deuteride, while the high-field H-l NMR shows two distinct triplets at t42.3 and r42.4, assignable to H2IrCl(PCy3)2 and HDIrCl(PC3)2, respectively. 

Mrugala F, Moszynski R (1998) Near-infrared absorption spectrum of the Ar-HD complex Confrontation of theory with experiment. J Chem Phys 109 10823-10837... 

This scrambling could be established by NMR spectroscopy. Measurement of the T relaxation time further served to identify the rp-HD complex [Ru(q2-HD)(PCy3)(S4)] as an intermediate. /(HD) = 32 Hz, 7) (min) = 4 ms] (Fig. 20) (144). In the case of rhodium, the corresponding complex [Rh(r 2-HD)(PCy3)-(S4) 1 is cationic and presumably represents a transition state that could not be detected by NMR spectroscopy. 

Since H atoms are not accurately located by X-ray diffraction, and neutron diffraction studies cannot be carried out on a routine basis, spectroscopic methods have proved most useful in this area. The best way to characterize dihydrogen complexes relies on NMR spectroscopy a large J(H,D) of the corresponding HD complex reveals the presence of the H-D sigma bond. 

Zibn s solid-state powder NMR method, which measures the H H dipolar coupling, is less affected by motional problems and may give a better estimate of c hh Ligand deuteriation is, however, usually ueeded for optimum results. The kind of accuracy that can be achieved is illustrated by the for Kubas complex, 0.890 0.006 A. In the absence of neutron data, X-ray diffraction has been used but its reliability is questionable for H2 complexes. If J(H,D) is observable in the corresponding HD complex, Morris equation can be used. This reliably relates the J(H,D) value directly to ( hh (and to the bond order). 

An )] -HD complex has an IR spectrum that is not simply a superposition of that for the r] -T>2 as would be the case for the classical M(H)2 analogues. Instead v(H-D) of the HD complex appears approximately halfway between the corresponding v(H-H) and v(D-D). The v(H2) stretching band, typically appearing near 2700 cm, does not correlate with binding strength because of extensive mixing of the many modes. 

Ultimately,1H NMR spectroscopy turned skeptics into believers the HD complex W(CO)3(P Pr3)2(HD) was synthesized and showed a large HD coupling constant that proved that the coordinated H-D bond was mostly intact and not split to give a hydride-deuteride such as W(H)(D)(CO)3(P Pr3)2. The NMR showed a 1 1 1 triplet (deuterium spin=l) with Aid 33.5 Hz, nearly as high as in HD gas, 43.2Hz. Observation of Jhd higher than that for a dihydride complex (>2 Hz) became the premier criterion for a H2 complex, as will be described. 

Figure 5.6. Plot of 4 determined in the solid state versus measured for the corresponding HD complex in solution. Reprinted with author s permission from Morris, R. H. Can. J. Chem. 1996, 74,1907.

Figure 8.1. Ranges and shifts observed for vibrational modes in HH, HD, and DD complexes (INS = inelastic neutron scattering). The D2 and HD complexes are primarily M(CO)3(PR3)2(H2) (M = group 6).

We showed that the H-H distances from these and other structures correlate linearly with the coupling constant J(HD) measured from the H NMR spectrum of the HD analogue in solution (eq 10) [30]. This equation is now an accepted method for determining the H-H distance of new dihydrogen complexes from the H NMR spectrum of the HD complex in solution, such as the very long distance of 1.38 A in fac-[Re(H-H)(dien)(PPh3)(PF3)]+ [35]. 

---