Abstract

Cadmium bromide (CdBr₂) is an inorganic compound with a molar mass of 272.22 g/mol that crystallizes as a white hygroscopic solid. The compound exhibits a rhombohedral crystal structure with space group R-3m (No. 166) and demonstrates significant solubility in polar solvents including water, ethanol, ether, and acetone. Cadmium bromide melts at 568°C and boils at 844°C with a density of 5.192 g/cm³ in its solid state. The tetrahydrate form (CdBr₂·4H₂O) possesses a polymeric structure with bridging bromide ligands and interstitial water molecules. Despite its well-characterized physical and chemical properties, cadmium bromide finds limited industrial applications due to the toxicity of cadmium compounds and is primarily utilized in research contexts and specialized photographic processes.

Introduction

Cadmium bromide represents a classical example of a metal halide compound with the chemical formula CdBr₂. Classified as an inorganic salt, it belongs to the broader category of cadmium halides alongside cadmium chloride and cadmium iodide. The compound exists primarily in anhydrous and hydrated forms, with the tetrahydrate (CdBr₂·4H₂O) being the most common hydrated species. Cadmium bromide demonstrates characteristic properties of ionic compounds including high melting point, electrical conductivity in molten state, and solubility in polar solvents. The compound's significance lies primarily in its structural chemistry and as a precursor in various synthetic pathways, though its practical applications remain limited due to environmental and toxicological concerns associated with cadmium compounds.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Cadmium bromide adopts a rhombohedral crystal structure with space group R-3m (No. 166) in its anhydrous form. The cadmium centers exhibit octahedral coordination geometry with six bromide ligands arranged in a regular octahedral configuration. Each cadmium atom (electron configuration [Kr]4d¹⁰5s⁰) exists in the +2 oxidation state, while bromide ions possess the electron configuration [Ar]3d¹⁰4s²4p⁶. The compound demonstrates ionic bonding character with partial covalent contribution due to polarization effects. Bond lengths between cadmium and bromide average approximately 2.69 Å in the crystalline state. The electronic structure shows a filled valence band primarily composed of bromide 4p orbitals and a conduction band dominated by cadmium 5s orbitals, resulting in a band gap of approximately 3.5 eV.

Chemical Bonding and Intermolecular Forces

The bonding in cadmium bromide is predominantly ionic with significant covalent character arising from polarization of the bromide ions by the small, highly charged Cd²⁺ cation. The compound's lattice energy measures approximately 2150 kJ/mol, calculated using the Born-Mayer equation. Intermolecular forces in the solid state include strong electrostatic interactions between cations and anions, with van der Waals forces contributing minimally due to the ionic nature of the compound. The tetrahydrate form exhibits additional hydrogen bonding between water molecules and bromide ions, with O-H···Br distances measuring approximately 2.8 Å. The molecular dipole moment in gaseous CdBr₂ measures 10.3 D, reflecting the significant charge separation in this predominantly ionic compound.

Physical Properties

Phase Behavior and Thermodynamic Properties

Cadmium bromide appears as a white crystalline solid with a density of 5.192 g/cm³ at 25°C. The compound melts at 568°C and boils at 844°C under standard atmospheric pressure. The enthalpy of fusion measures 21.5 kJ/mol, while the enthalpy of vaporization is 145 kJ/mol. The specific heat capacity at constant pressure (Cp) is 75.3 J/mol·K at 298 K. Cadmium bromide exhibits high solubility in water: 56.3 g/100 mL at 0°C, 98.8 g/100 mL at 20°C, and 160 g/100 mL at 100°C. The compound is also soluble in ethanol, diethyl ether, acetone, and liquid ammonia. The magnetic susceptibility measures -87.3×10⁻⁶ cm³/mol, indicating diamagnetic behavior consistent with the d¹⁰ electronic configuration of Cd²⁺.

Spectroscopic Characteristics

Infrared spectroscopy of cadmium bromide reveals characteristic Cd-Br stretching vibrations at 210 cm⁻¹ and 185 cm⁻¹ in the solid state. Raman spectroscopy shows a strong band at 165 cm⁻¹ corresponding to the symmetric stretching vibration of the Cd-Br bond. Ultraviolet-visible spectroscopy demonstrates an absorption edge at 355 nm corresponding to the charge-transfer transition from bromide to cadmium. Mass spectrometric analysis of vaporized CdBr₂ shows predominant peaks at m/z 272 (CdBr₂⁺), 192 (CdBr⁺), and 80 (Br⁺) with characteristic isotopic patterns reflecting the natural abundance of cadmium isotopes (¹¹⁴Cd: 28.7%, ¹¹²Cd: 24.1%, ¹¹⁰Cd: 12.5%, ¹¹³Cd: 12.2%, ¹¹⁶Cd: 7.5%, ¹¹⁸Cd: 5.7%).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Cadmium bromide demonstrates typical reactivity patterns of ionic cadmium compounds. The compound undergoes double displacement reactions with soluble sulfides to form bright yellow cadmium sulfide precipitate with a solubility product constant (Ksp) of 8.0×10⁻²⁷. Reaction with hydroxide ions produces cadmium hydroxide precipitate (Ksp = 7.2×10⁻¹⁵). Cadmium bromide participates in complex formation reactions with ammonia, cyanide, and halide ions, forming stable complexes such as [Cd(NH₃)₄]²⁺ (log β₄ = 7.0), [Cd(CN)₄]²⁻ (log β₄ = 18.9), and [CdBr₄]²⁻ (log β₄ = 2.6). Thermal decomposition occurs above 900°C, producing cadmium vapor and bromine gas. The compound is stable in dry air but gradually hydrolyzes in moist air to form basic bromides.

Acid-Base and Redox Properties

Cadmium bromide solutions exhibit weakly acidic behavior due to hydrolysis of the Cd²⁺ ion, with pH values typically ranging from 5.5 to 6.5 for saturated aqueous solutions. The hydrolysis constant (Kh) for [Cd(H₂O)₆]²⁺ is 3.2×10⁻⁹ at 25°C. The standard reduction potential for the Cd²⁺/Cd couple is -0.40 V versus the standard hydrogen electrode, indicating moderate reducing power. Cadmium bromide does not undergo significant oxidation under ambient conditions but can be oxidized by strong oxidizing agents such as fluorine or chlorine at elevated temperatures. The compound demonstrates stability in neutral and weakly acidic conditions but decomposes in strongly acidic media through proton-assisted pathways.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most direct laboratory synthesis involves the reaction of cadmium metal with bromine vapor at elevated temperatures (300-400°C). The reaction proceeds according to the equation: Cd(s) + Br₂(g) → CdBr₂(s) with nearly quantitative yield. Alternative synthetic routes include the reaction of cadmium carbonate or cadmium oxide with hydrobromic acid: CdCO₃(s) + 2HBr(aq) → CdBr₂(aq) + CO₂(g) + H₂O(l). The tetrahydrate form crystallizes from aqueous solution below 36°C through slow evaporation or cooling of saturated solutions. Purification typically involves recrystallization from water or ethanol, followed by drying under vacuum at 100°C to obtain the anhydrous compound. The anhydrous form can also be prepared by dehydration of the hydrate using thionyl chloride or by heating under vacuum at 200°C.

Analytical Methods and Characterization

Identification and Quantification

Qualitative identification of cadmium bromide employs precipitation tests with sulfide ions (yellow precipitate), hydroxide ions (white precipitate soluble in excess ammonia), and chromate ions (yellow precipitate). Quantitative analysis typically utilizes atomic absorption spectroscopy with detection limits of 0.01 mg/L for cadmium and 0.1 mg/L for bromide. Inductively coupled plasma mass spectrometry provides lower detection limits of 0.1 μg/L for cadmium determination. Bromide content can be determined gravimetrically as silver bromide or titrimetrically using Volhard's method. X-ray diffraction provides definitive identification through comparison with reference patterns (ICDD PDF #00-007-0207 for anhydrous CdBr₂). Thermal analysis techniques including thermogravimetry and differential scanning calorimetry characterize hydration states and phase transitions.

Purity Assessment and Quality Control

Commercial cadmium bromide typically assays at 99.0-99.9% purity with common impurities including chloride, sulfate, and iron. Determination of chloride impurity employs potentiometric titration with silver nitrate, while sulfate content is determined turbidimetrically as barium sulfate. Trace metal impurities are analyzed using atomic absorption spectroscopy or inductively coupled plasma optical emission spectroscopy. The compound should be stored in airtight containers protected from moisture due to its hygroscopic nature. Stability testing indicates no significant decomposition under proper storage conditions for periods exceeding five years.

Applications and Uses

Industrial and Commercial Applications

Cadmium bromide finds limited industrial application due to toxicity concerns. Historical uses included photographic processes as a sensitizer for collodion plates and in lithography. The compound serves as a precursor for the preparation of other cadmium compounds, particularly cadmium sulfide pigments through precipitation reactions. Cadmium bromide functions as a catalyst in certain organic transformations including the Reformatsky reaction and aldol condensations. The compound has been employed in electroplating baths for cadmium deposition, though this application has declined due to environmental regulations. Laboratory uses include as a bromide source in synthetic chemistry and as a component in low-melting point alloys when combined with other metal bromides.

Historical Development and Discovery

Cadmium bromide was first prepared in the early 19th century following the discovery of cadmium metal in 1817 by German chemists Friedrich Stromeyer and Karl Samuel Leberecht Hermann. Early investigations focused on establishing its chemical composition and basic properties. The compound's crystal structure was determined using X-ray diffraction techniques in the 1920s, revealing the rhombohedral arrangement. Research throughout the mid-20th century elucidated its coordination chemistry and complex formation tendencies. Environmental and toxicological concerns emerging in the latter half of the 20th century significantly reduced industrial applications of cadmium compounds including cadmium bromide. Recent research has focused on understanding its fundamental solid-state properties and potential applications in materials science.

Conclusion

Cadmium bromide represents a well-characterized inorganic compound with distinctive structural and chemical properties. Its rhombohedral crystal structure, high solubility in polar solvents, and predictable reactivity patterns make it a model compound for studying cadmium chemistry. The compound's limited practical applications reflect growing concerns regarding cadmium toxicity and environmental impact. Future research directions may include exploration of its potential in photovoltaic materials, semiconductor applications, and as a precursor for nanostructured cadmium chalcogenides. Fundamental studies continue to investigate its phase behavior, spectroscopic characteristics, and coordination chemistry with various ligands.