Metal carbonyl hydrides

Cobalt carbonyl hydride

Metal carbonyl hydrides are complex compounds of transition metals with carbon monoxide and hydrogen as ligands. The compounds are used in organic synthesis and as catalysts in homogeneous catalysis , for example in hydroformylation .

presentation

Walter Hieber succeeded in preparing the first preparation of a metal carbonyl hydride in 1931 when he investigated the so-called Hieber base reaction of metal carbonyls . A hydroxide ion reacts with metal carbonyls such as iron pentacarbonyl with nucleophilic attack on the carbonyl carbon atom, releasing carbon dioxide and forming metal carbonylates. In a further step, iron carbonyl hydride, a very temperature and air-sensitive complex hydride, can be synthesized. The preparation of cobalt carbonyl hydride (HCo (CO) ₄ ) succeeds in the same way.

${\displaystyle {\ce {Fe(CO)5{}+NaOH->Na[Fe(CO)4COOH]->[{\text{+ NaOH}}]Na[HFe(CO)4]{}+NaHCO3}}}$

${\displaystyle {\ce {Na[HFe(CO)4] + H+ -> H2[Fe(CO)4] + Na+}}}$

Another synthetic route is the reaction of metal carbonyls with hydrogen. The protonation of metal carbonylates also leads to metal carbonyl hydride.

properties

Metal carbonyl hydride	pK s
HCo (CO) 4	"Strong acid"
HCo (CO) 3 (P (OPh) 3 )	5.0
HCo (CO) 3 (PPh 3 )	7.0
HMn (CO) 5	7.1
H 2 Fe (CO) 4	4.4, 14
[HCo ( dmgH ) 2 PBu 3 ]	10.5

In metal carbonyl hydrides, the hydrogen atom is bonded directly to the metal. For cobalt carbonyl hydride, for example, the metal-hydrogen distance is 114 pm, the metal-carbon distance for the axial CO ligand is 176 pm and for the equatorial CO ligands 182 pm. The neutral metal carbonyl hydrides are often volatile and react acidic in aqueous solution.

use

Metal carbonyl hydrides are used as catalysts for hydroformylation. The reaction begins with the generation of a coordinatively unsaturated metal carbonyl hydride complex such as HCo (CO) ₃ or HRh (CO) (PPh ₃ ) ₂ by splitting off a carbon monoxide ligand. Such species initially bind alkenes through π bonding. An alkyl complex is formed by insertion into the metal-hydrogen bond. Carbon monoxide can insert into the metal-carbon bond of the alkyl radical. The starting metal carbonyl hydride complex is formed again by oxidative addition of hydrogen and the subsequent elimination of the aldehyde from the complex.

Analytical characterization

For a long time it was uncertain whether the metal carbonyl hydrides have a direct metal-hydrogen bond, although this was already suspected by Hieber. The exact structure could not be identified by X-ray diffraction , in particular the length of a possible metal-hydrogen bond remained unclear. The exact structure of the metal carbonyl hydrides was determined primarily by means of neutron diffraction and nuclear magnetic resonance spectroscopy .

literature

• AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 .

Web links

Commons : Metal Carbonyl Hydrides  - Collection of Images, Videos, and Audio Files

Individual evidence

1. ↑ W. Hieber, F. Leutert: To the knowledge of the coordinatively bound carbon oxide: formation of iron carbonyl hydrogen. In: The natural sciences. 19, 1931, pp. 360-361 ( doi : 10.1007 / BF01522286 ).
2. ↑ HD Kaesz, SAR Knox, JW Koepke, RB Saillant: Synthesis of metal carbonyl hydrides from metal carbonyls and hydrogen at atmospheric pressure. In: Journal of the Chemical Society D: Chemical Communications. 1971, p. 477 ( doi : 10.1039 / C29710000477 ).
3. ^ ^{A b} E. A. McNeill, FR Scholer: Molecular structure of the gaseous metal carbonyl hydrides of manganese, iron, and cobalt. In: Journal of the American Chemical Society. 99, 1977, pp. 6243-6249 ( doi : 10.1021 / ja00461a011 ).
4. ^ Ralph G. Pearson: The transition-metal-hydrogen bond. In: Chemical Reviews. 85, 1985, pp. 41-49 ( doi : 10.1021 / cr00065a002 ).
5. ↑ FA Cotton: Structure and Bonding in Metal Carbonyls and Related Compounds. In: Helvetica Chimica Acta. 50, 1967, pp. 117-130 ( doi : 10.1002 / hlca.19670500910 ).