Classical Transition Metal Hydrides

In low-temperature X-ray analysis and in neutron diffraction studies the structure of solid HMn(CO)5 was elucidated [9]. The arrangement of the ligands around the Mn atom is octahedral with the four equatorial carbonyl groups somewhat bent toward the axial hydrogen atom. This proves that hydrogen is a small but stereo-chemically active ligand, a fact which had been questioned previously [10]. 

Among the classical hydride complexes there are not only monohydrides and dihydrides, but also polyhydrides. Well-known trihydrides are ( -C5H5)2NbH3 

In 1965 the compound RhCl(PPh3)3, coined the Wilkinson catalyst , was shown to be an excellent hydrogenation catalyst [17-19]. Although there had been scattered reports on hydrogenation catalysts before [20], it was this discovery which started a worldwide activity in the field of hydrogenation reactions (vide infra). 

---

Solution and solid state studies of non-classical transition metal hydride systems have revealed fast intramolecular motions of the dihydrogen ligands [7, 8]. These motions represent a rotational diffusion around the axis perpendicular to the H-H (or D-D) vector (Scheme 1), a libration (Scheme 2) or 180° - jumps (Scheme 3) around the same axis [43]. It has been shown that the type of motion, its frequency and the orientation of eq z (angle a in Scheme I) affect strongly on the 2H NMR parameters, causing an elongation of the H T, time in solution [25] or a decrease of the quadrupole splitting in the solid -state H NMR spectra [22]. This influence creates additional problems for DQCC determinations from experimental relaxation measurements or solid state NMR data, particularly when the orientation of the eqzz vector is unknown. 

The unusual H-H bonding mode in dihydrogen complexes is of the greatest interest, also from the point of view of DQCC measurements. MO calculations of a simple [Rb-Dj]" model have shown that a transformation of classical dihydride structures to dihydrogen complexes leads to a dramatic increase of DQCC from 50 up to 155 kHz [11]. In addition, the asymmetry parameter T) grows from 0.025 to 0.62 and the orientation of the major axis of EFG (eq ) is remarkably deviated from the D-D vector in the dihydrogen complex (Scheme 1). Similar data have also been obtained in MO calculations of transition metal hydrides (Table 6). 

The association of transition metal complexes and of hydrides offer a convenient method for modifying the selectivities (and efficiencies) of the classical hydride reagents. Ill-defined, very active reducing species are formed (transition-metal hydrides ) that promote selectivities quite different from those of the corresponding non-catalyzed reactions. 

Particular transition metal hydrides have found very promising applications in organic reactions, as is shown for example by the hydrozircona-tion reaction, which appears as a remarkable improvement of the classical hydroboration reaction (the high cost of the reagent, however, limits its application in large scale preparations). 

The classical synthetic methods to generate transition metal hydrides are based on (i) the / -hydrogen elimination from a methoxide complex, usually generated in situ by treatment of a chloride precursor with Na[OMe] or K[OMe], and (ii) halide/hydride metathesis using NalBHJ or Li[AlH4]. Following these approaches, the novel monohydride derivatives have been synthesized ... 

This special feature arises from the combination of the transition metal behavior such as the coordination of a carbon-carbon multiple bond, oxidative addition, reductive elimination, P-hydride elimination, addition reactions and the behavior of classical c-carbanion towards electrophiles. 

Recent interest has been focused more on the formation of distannanes by treatment of tin hydrides with transition metals, rather than on these more classic methods.443... 

It is known that the polymerization of ethylene by trialkyl aluminum is not a rapid reaction at normal pressures and temperatures. Ziegler, Gellert, Holzkamp, Wilke, Duck and Kroll (72) have found that ethylene was polymerized to higher trialkylaluminums only at elevated temperatures and pressures. Anionic hydride transfer commonly occured under these conditions. However, the addition of a transition metal halide such as titanium tetrachloride, the classical Ziegler catalyst, polymerized ethylene rapidly under mild conditions. 

In most cases the product will be the classical metal hydride (21-VIc), with structures like (21-VIa) or (21-VIb) as transition states. Increasingly often, however, it is observed that the reaction does not proceed to the oxidative addition product but gives complexes of dihydrogen (21-VIa) or elongated dihydrogen (21-VIb) instead. 

Certain classical coordination complexes (see Coordination Complexes) of iron (e.g. Prussian blue) will be dealt with in other articles (see Iron Inorganic Coordination Chemistry and Cyanide Complexes of the Transition Metals), as will much of the chemistries of iron carbonyls (see Metal Carbonyls) and iron hydrides (see Hydrides) (see Carbonyl Complexes of the Transition Metals Transition Metal Carbonyls Infrared Spectra, and Hydride Complexes of the Transition Metals). The use of organoiron complexes as catalysts (see Catalysis) in organic transformations will be mentioned but will primarily be covered elsewhere (see Asymmetric Synthesis by Homogeneous Catalysis, and Organic Synthesis using Transition Metal Carbonyl Complexes). 

In this case, there is no reason to suppose that hydrogen is coordinated in any way other than the classical dihydride manner (30) see Hydrides Solid State Transition Metal Complexes). However, similar experiments using Cr(CO)5 gave a product Cr(CO)5H2 for which strong circumstantial evidence pointed towards the nonclassical dihydrogen structure (31). Unfortunately, in low-temperature matrices... 

Aliphatic iodides, and especially secondary and tertiary representatives, are subject to hydride ehmination and are not generally useful substrates in transition metal catalysed coupling reactions. The last reaction in Scheme 14 [31], for instance, cannot be executed using current transition metal-based technology. In contrast, vinyl and aryl iodides, which are superb partners in many classical metal-induced couphng reactions, are very poor substrates in the present radical process because of the high energy of vinyl and aryl radicals. The two methods thus nicely complement each other. 

---