Non-classical hydrides

Due to localization of the electron density, Scheme 10, the oxidation state of the metal increases by two units upon passing from a non-classical to a classical coordination, which in principle offers the opportunity to use electrochemistry to discriminate between classical and non-classical hydrides.81... 

The first attempt to use electrochemistry to solve the dilemma of classical or non-classical coordination was in the case of the polyhydride [ReH5(PPh3)3]. Its solid-state molecular structure would correspond to that of the classical pentahydride [RevH5(PPh3)3], whereas solution NMR studies would indicate a non-classical hydride [Re,nH3(H2)(PPh3)3]. 

Unfortunately, as rhenium can assume all the oxidation states in the range from -1 to + 7, such an anodic process can be attributed either to the oxidation Re(V)/Re(VI), according to the classical hydride formulation [RevH5(PPh3)3], or to the Re(III)/Re(IV) oxidation, according to the non-classical hydride formulation [RemH3(H2)(PPh3)3]. 

In this regard a more significant determination should be obtained from a study of the polyhydride complexes [ReH7(PR3)2], in that (i) if the complex is a classical hydride the rhenium atom would be in its maximum oxidation state (+ 7) and, therefore, could not undergo further oxidation processes (ii) if the complex is a non-classical hydride, the rhenium atom would be in a lower oxidation state and, consequently, could display oxidation processes. 

This observation (coupled to NMR data) seemed to support the thesis that the complex was a non-classical hydride. (Unfortunately) the neutron diffraction structure of a few related complexes such as... 

Furthermore, this demonstrated that the factors governing the equilibrium between classical and non-classical hydrides are very delicately balanced. 

Let us now consider a few non-classical hydride complexes for which molecular structures are available. 

As already mentioned in Chapter 7, Section 2 (Figures 18, 19), the monocation [Co(PP3)(H2)]+ (PP3 = P(CH2CH2PPh2)3), present as non-classical hydride in [Co(PP3)(H2)][PF6], undergoes an irreversible oxidation (Figure 19), which generates the monocation [Co(PP3)H]+, thus suggesting that the oxidation of the dihydrogen complex simply causes deprotonation.83... 

It is useful to note that the simple substitution of the counteranion to form [Co(triphos)(H2)][BPh4] produces a classical hydride complex. This is a further demonstration of the lability which characterizes the boundary between classical and non-classical hydrides. 

In CH2C12 solution this complex displays a reversible one-electron oxidation at E° = 0.0 V (vs. SCE).90 The easy oxidation agrees well with the Re(I) oxidation state in a non-classical hydride formulation, even if the calculated H-H bond distance of 1.39 A poses the derivative at the border of classical and non-classical hydrides.91... 

Regio- and stereo-selective dimerization of alk-l-ynes catalysed by classical and non-classical hydride complexes of Ru(II) and Os(II) stabilized by the tripodal polydentate ligand (Pl PCH CH P has been reported to produce the corresponding (Z)-1,4-disubstituted butenynes. Irrespective of the nature of the hydride ligand (classical or non-classical), vinylidene complexes appear to be the immediate precursors to the C—C bond-forming step.67... 

The nonclassical hydride complex, W(i7 -H2)(CO)3(PCy3)2, exists in solution in equilibrium with its classical tautomer W(H)2(CO)3(PCy3)2 . Similar equilibria have been observed for di- and polyhydrides -. Nonclassical hydride ligands must certainly exist in equilibrium with some catalytically active classical hydride complexes. As intermediates to more reactive classical hydrides, non-classical hydrides would simply tie up available catalyst. 

Normally, most hydride resonances have relatively long relaxation times[12] with only non-classical hydrides displaying short relaxation times due to the close proximity of the two H-nuclei in the coordinated Hj molecule.[13] However no example of this class of T 2-H2 coordination has yet been reported for cluster compounds, and this possibility will not be further considered here. 

Rapid conversion between C-Ir/H-C 19 and C-H/lr-C 21 is observed, a reaction that involves oxidative addition of the agostic C-H bond and reductive elimination of the aryl hydride. This is a rare example of reversible metalation of an agostic group under mild conditions. The tautomeric equilibration was suggested to go via the doubly metalated species 20 having a non-classical hydride but an Ir(V) dihydride could not be excluded. 

Tp RhH2(H2) (748) prepared from [PPh4][Tp RhCl3] and NaBH4, which was the first example of a non-classical hydride stabihsed only by nitrogen... 

The role of molecular hydrogen complexes in homogeneous catalysis has been reviewed by Oro and Esteruelas and by Bianchini and Peruzzini. The work includes a whole coverage, up to the year 2000, of the literature on transition metal non-classical hydrides, including iridium, which are thought to take part in catalytic reactions. 

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