Hydride classical

All the compounds of the type RuH2(PR3)4 seem to be classical hydrides... 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

All the compounds of the type RuH2(PR3)4 seem to be classical hydrides RuC13.xH20----------------------—------ mww-RuH2(PPh3)4... 

Protonation of IrH5(Pcy3)2 affords a fluxional complex thought to be IrH6(Pcy3)2 with two (7y2-H2) ligands and two classical hydrides (Figure 2.99) from the H NMR (6 = -8.3 ppm at room temperature ... 

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... 

---