Hydride and Dihydrogen Complexes

Burdett, O. Eisenstein, and S. A. Jackson, Transition Metal Dihydrogen Complexes Theoretical Studies, In Transition Metal Hydrides, A. Dedieu, Ed., VCH Publishers New York, 1992. 

One of the most interesting aspects of transition metal hydride chemistry is the relationship between this ligand and the rapidly developing chemistry of the dihydrogen ligand, H2. 

Although complexes containing H2 molecules coordinated to transition metals had been proposed for many years and many complexes containing hydride ligands had been prepared, the first structural characterization of a dihydrogen complex did not occur until 1984, when Kubas and co-workers synthesized the complexes M(CO)3(PR3)2(H2) (M = Mo, W R = cyclohexyl, isopropyl).30 Subsequently, many H2 complexes have been identified, and the chemistry of this ligand has developed rapidly. 

Exercise 6-3 Explain why Mo(PMe3)5H2 is a dihydride (contains two separate 

The dividing line between whether an M(H)2 or M(H2) structure will be favored is a narrow one, and subtle differences in environment can be important. For example, in Rh[P(CH2CH2PPh2)3](H2) +, two isomers are found, as illustrated below. If hydrogen occupies an apex of the trigonal bipyramidal isomer, it is present as H2 if hydrogen occupies cis positions of the octahedral isomer, it is present as separate H atoms.31 Calculations have demonstrated that there is less donation from the metal to the ligand at the apical site of the trigonal bipyramid than in the cis positions of the octahedral isomer.32 

---

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

Preparation and reactivity with azo-species of hydride and dihydrogen complexes of osmium stabilised by tris(pyrazolyl)borate and phosphite ligands... 

X-ray diffraction is a standard method employed by chemists to determine the structure of new complexes. In the case of transition metal hydride and dihydrogen complexes, precise location of metal-bound hydrogen atoms by X-ray diffraction is very difficult. Superior structural information is provided by neutron diffraction, but the requirement for large well-formed single crystals has limited this method to a small subset of the known complexes.118... 

The synthesis and characterization of hydride and dihydrogen complexes with the widest range possible of acid-base reactivity... 

THE SYNTHESIS AND CHARACTERIZATION OF HYDRIDE AND DIHYDROGEN COMPLEXES WITH THE WIDEST RANGE POSSIBLE OF ACID-BASE REACTIVITY... 

There are several other results that could be put in the category of variations of standard conditions, including other ligands and other metals. These are covered in Section 2.7. Before we turn to these variations we want to explore the issues surrounding hydride and dihydrogen complexes. 

---