Rare‐Earth Tetracyanidoborate Salts – Structural Features and Properties Including Luminescence of [ RE (H 2 O) 8 ][B(CN) 4 ] 3 · n H 2 O and [ RE (H 2 O) 7 {κ 1 N ‐B(CN) 4 }][B(CN) 4 ] 2 ( RE = Y, Tb, Dy, Ho, Er, Tm, Yb, Lu; n ≤ 3)

New tetracyanidoborate salts with hydrated rare‐earth‐metal cations ( RE 3+ ) were synthesized by the reactions of rare‐earth‐metal oxides or hydroxides with tetracyanidoboronic acid, H[B(CN) 4 ] · n H 2 O. Two groups of isostructural compounds were obtained with the formulas [ RE (H 2 O) 8 ][B(CN) 4 ] 3 · n H 2 O (structure I) and [ RE (H 2 O) 7 {κ 1 N ‐B(CN) 4 }][B(CN) 4 ] 2 (structure II) for RE = Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu and n ≤ 3. Both structures have discrete eight‐coordinate RE 3+ complex cations, as [ RE (H 2 O) 8 ] 3+ in structure I and [ RE (H 2 O) 7 {κ 1 N ‐B(CN) 4 }] 2+ in structure II. The heating of the compounds to temperature of ca. 160 °C gives anhydrous samples, but elongated heating to temperatures above ca. 200 °C leads to the slow decomposition of the tetracyanidoborates. The luminescence properties of a solid sample of the anhydrous Tb salt and a solution of the Dy salt were investigated. Both show intense lines in their excitation and emission spectra that were assigned to electronic transitions within the f and d shells.