Kubas complexes

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). 

As noted above, a continuum of values for dHH is now available, ranging from very short as in the original Kubas complexes to conventional dihydride complexes with 1.5 A. A relatively small number of complexes with intermediate values of dHH known as elongated dihydrogen complexes have been reported.14 An example of this type of complex is provided by [Cp Ru(dppm)(H2)]+. A value of dHH = 1.10 . 03 A was reported using neutron diffraction by Morris, Koetzle, and coworkers in 1994 (Figure 5.2).122... 

More recent work from the NREL group, ° in which they compare DET and quantum Monte Carlo results for binding energies in systems that are similar to those discussed above, shows that the DET binding energies reported can be expected to be accurate to within 0.1 eV. This is not a surprise, as the bonding in Kubas complexes is, in essence, of a mixed ionic-covalent character and such situations are described with acceptable accuracy within DET. 

Finally, Kubas complexes are discussed in detail in Ref. 47 which also includes references to the relevant literature. 

The discovery of the Kubas complex was a defining event in the historical development of coordination chemistry. By 1920, the Lewis ideas on the role of electron pairs in bonding had already associated the coordinate bond with the donation of a lone pair to a metal. For example, donation of the ammonia lone pair to was implicated in the classical Werner cobalt-ammonia complexes. Subsequent developments extended the coordination concept beyond lone-pair donors. Around 1950, a series of discoveries by Wilkinson, Chatt, Fischer, and others showed how the electrons of unsaturated ligands such as cyclopentadienide ion and ethylene can also bind to metal ions. These complexes stimulated the modem development of organometallic chemistry and homogeneous catalysis. 

Some new discoveries have implications that are worked out within a few years. Others, like that of the Kubas complex, lead to rich veins of intellectual ore that continue to produce for years to come and forge links among disparate areas of science. Those of us who are interested in metal-hydrogen chemistry therefore live in a fortunate time. 

In solution, an equilibrium is established for the Kubas complex between the dihydrogen and dihydride forms, confirming that the molecular hydrogen complex is an intermediate in the oxidative addition reaction of H2. 

In the Kubas complex, the presence of carbonyl ligands (jt acceptors) and phosphines (which may, in a first approximation, be considered as... 

We note, in conclusion, that the conformation adopted by H2 in the Kubas complex is controlled by the back-donation interaction (4-15). 

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