Rare‐Earth Metal Tetracyanidoborate Hydrate Salts: Structural, Spectral, and Thermal Properties as well as the Luminescence of Dehydrated Salts

New tetracyanidoborate salts with hydrated rare‐earth metal cations ( RE 3+ ) were obtained by the reaction of rare‐earth metal oxides or hydroxides with tetracyanidoboronic acid. A group of isostructural compounds was obtained, which has the chemical formula [ RE {B(CN) 4 } 3 (H 2 O) 5 ] · n H 2 O ( RE = La, Ce, Pr, Nd, Sm, Eu, and Gd; n ≤ 0.5). In all compounds the rare‐earth metal cations have the coordination number nine with a RE ‐O 5 N 4 environment. They are connected to chains through one bridging tetracyanidoborate anion per RE 3+ cation, according to [ RE {μ‐B(CN) 4 }{κN‐B(CN) 4 } 2 (H 2 O) 5 ] · n H 2 O. Solid samples of the Eu salt show luminescence with intense lines in the excitation and emission spectra caused by electronic transitions within the f and d shells.Thermal investigations indicate that all water molecules but one can be removed at temperatures below 160 °C and the last one by heating up to 340 °C. On further heating decomposition is observed.