- Cadmium hydride (CdH₂) is an inorganic compound consisting of cadmium in the +2 oxidation state bonded to hydride anions (H⁻).
- It is a white to gray solid that is highly unstable under normal conditions and is known primarily as a laboratory curiosity rather than a commercially useful substance.
- Like other alkaline-earth and transition-metal hydrides, cadmium hydride is an ionic compound at the macroscopic scale but contains significant covalent character in its cadmium–hydrogen bonds. Due to its instability and dangerous decomposition products, CdH₂ is seldom handled outside of specialized research settings.
- CdH₂ is generally prepared by the direct reaction of cadmium metal with hydrogen gas under elevated pressures (typically above 200 atmospheres) and moderate heating, or by the metathesis reaction between cadmium(II) halides and alkali metal hydrides such as lithium hydride (LiH) or sodium hydride (NaH). The compound forms as a crystalline powder that is sensitive to heat, light, and mechanical shock. Because it decomposes readily, cadmium hydride is not easily stored, and samples are usually prepared in situ for immediate use.
- Chemically, cadmium hydride is thermodynamically unstable and decomposes upon mild heating or even at room temperature over time, producing cadmium metal and hydrogen gas:
- CdH₂ → Cd + H₂↑
- This decomposition is exothermic, and in finely divided form, CdH₂ may ignite spontaneously due to the heat released. It reacts violently with oxidizing agents and can also react with moisture, although its instability often precludes observation of slow hydrolysis.
- From a practical standpoint, cadmium hydride’s main relevance lies in academic research on metal hydrides, hydrogen storage materials, and the bonding behavior of post-transition metals with hydrogen. Unlike some lighter metal hydrides (e.g., magnesium hydride), CdH₂ is not a viable hydrogen storage medium because of its low thermal stability and high toxicity. Additionally, cadmium’s relatively large ionic radius and low electronegativity contribute to weaker metal–hydrogen bonding, which explains the compound’s easy decomposition.
- Toxicity is a critical hazard for cadmium hydride. Upon decomposition, it releases finely divided cadmium metal dust, which is highly toxic if inhaled, and hydrogen gas, which is flammable and explosive. Any accidental formation of cadmium oxide from the metal dust during heating can produce highly irritating and poisonous fumes. Chronic cadmium exposure causes kidney damage, bone demineralization, and cancer, while acute exposure can lead to severe respiratory distress. For these reasons, cadmium hydride must be prepared and handled only in inert-atmosphere gloveboxes or sealed high-pressure systems, with all waste treated as hazardous.
- Environmentally, cadmium hydride poses risks due to its cadmium content. Even though the hydride form itself is short-lived in nature, any cadmium residues persist in soils and waterways, where they are toxic to plants, aquatic life, and humans through bioaccumulation. As a result, its production and use are tightly restricted, and it has no industrial-scale applications.
