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 (RE3+) 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(H2O)5]·nH2O (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‐O5N4 environment. They are connected to chains through one bridging tetracyanidoborate anion per RE3+ cation, according to [RE{μ‐B(CN)4}{κN‐B(CN)4}2(H2O)5]·nH2O. 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.