Rhodium carbonyl chloride
[CAS# 14523-22-9]

Identification

• Classification: Organic raw materials >> Organometallic compound >> Organic rhodium
• Name: Rhodium carbonyl chloride
• Synonyms: Di-�-chloro-tetracarbonyldirhodium(I); Rhodium carbonyl chloride dimer; Tetracarbonyl dichlorodirhodium
• Molecular Formula: C₄Cl₂O₄Rh₂
• Molecular Weight: 388.76
• CAS Registry Number: 14523-22-9 (14404-25-2)
• EC Number: 238-540-6
• SMILES: [C-]#[O+].[C-]#[O+].[C-]#[O+].[C-]#[O+].[Cl-].[Cl-].[Rh].[Rh]

Properties

• Melting point: 120-125 ºC (dec.) (Expl.)

Safety Data

• Hazard Symbols: GHS06;GHS07 Danger
• Hazard Statements: H301-H302-H330
• Precautionary Statements: P260-P264-P270-P271-P284-P301+P316-P301+P317-P304+P340-P316-P320-P321-P330-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute toxicity	Acute Tox.	4	H302

Discovory and Applicatios

Rhodium carbonyl chloride, with the empirical formula Rh(CO)₂Cl, is a rhodium(I) coordination complex containing two carbonyl (CO) ligands and one chloride ligand coordinated to a central rhodium atom. It typically exists in dimeric form under the formula \[Rh(CO)₂Cl]₂, where two rhodium centers are bridged by two chloride ligands, forming a four-membered Rh–Cl–Rh–Cl ring. This compound is of significant interest in organometallic and industrial chemistry due to its role as a precursor in catalytic processes and its well-defined structure.

The synthesis of rhodium carbonyl chloride commonly involves reacting rhodium(III) chloride hydrate with carbon monoxide gas under high pressure and elevated temperatures. The resulting product, \[Rh(CO)₂Cl]₂, is isolated as a yellow to orange crystalline solid. The use of carbon monoxide under controlled conditions reduces rhodium from the +3 to the +1 oxidation state while introducing CO ligands into the coordination sphere.

Structurally, each rhodium atom in \[Rh(CO)₂Cl]₂ is in the +1 oxidation state and adopts a square planar geometry, typical of low-spin d⁸ complexes. Each rhodium is bonded to two terminal carbonyl ligands and two bridging chloride ligands shared with the adjacent rhodium center. The CO ligands are strong field ligands that stabilize the metal center through synergic σ-donation and π-backbonding, which can be detected through characteristic infrared stretching frequencies in the range of 2000–2100 cm⁻¹.

Rhodium carbonyl chloride is primarily used as a precursor for the synthesis of other rhodium(I) complexes. It reacts readily with phosphine ligands, olefins, and other donor molecules to form catalytically active complexes. One of the most prominent derivatives is chlorocarbonylbis(triphenylphosphine)rhodium(I), RhCl(CO)(PPh₃)₂, which plays a key role in homogeneous catalysis.

In industrial applications, rhodium carbonyl chloride has been utilized in the hydroformylation of alkenes, a process that introduces a formyl group and hydrogen across a carbon–carbon double bond to yield aldehydes. These aldehydes serve as intermediates in the production of alcohols, plastics, and detergents. The compound’s ability to form reactive monomeric species in solution under catalytic conditions makes it a valuable intermediate in catalytic cycles involving oxidative addition and reductive elimination.

Beyond hydroformylation, \[Rh(CO)₂Cl]₂ has been used in the synthesis of complexes for hydrogenation, carbonylation, and C–H activation. In these transformations, the chloride and carbonyl ligands can be replaced by other functional groups or ligands to tailor the reactivity and selectivity of the rhodium center.

Handling rhodium carbonyl chloride requires caution due to its sensitivity to air and moisture and the toxic nature of carbon monoxide ligands. It should be stored under an inert atmosphere, such as nitrogen or argon, and handled in a fume hood with appropriate protective equipment. Decomposition can release carbon monoxide, a hazardous gas that can cause poisoning in enclosed spaces.

In summary, rhodium carbonyl chloride is a dimeric rhodium(I) complex featuring two carbonyl ligands and two bridging chloride ligands per metal center. It is synthesized by carbonylation of rhodium(III) salts and serves as a versatile intermediate in the preparation of catalytically active rhodium complexes. Its structure and reactivity underpin many applications in homogeneous catalysis and organometallic synthesis.