The First Boroselenates as new Silicate Analogues

The first boroselenates were obtained as single crystals by the reaction of selenic acid with boron acid and the corresponding alkali carbonates. The structure determinations showed a far‐reaching analogy to very recently described borosulfates and the borophosphates, that is, tetrahedral BO 4 and SeO 4 units linked by common corners. In each case, the BO 4 tetrahedra are surrounded by SeO 4 tetrahedra. As a function of the B/Se ratio, this results in chains (1:3; Cs 3 [B(SeO 4 ) 3 ], Rb 3 [B(SeO 4 ) 3 ]), isolated pentamers (1:4; HK 4 [(B(SeO 4 ) 4 ]), or pentamers with additional isolated SeO 4 tetrahedra (1:5; (H 3 O)Na 6 [B(SeO 4 ) 4 ](SeO 4 ). Compound Rb 3 [B(SeO 4 ) 3 ] (orthorhombic, Ibca , Z =8, a =7.508(2), b =15.249(3), c =23.454(5) Å) is isotypic to Rb 3 [B(SO 4 ) 3 ]) and Ba 3 [B(PO 4 ) 3 ]. Compound Cs 3 [B(SeO 4 ) 3 ] (monoclinic, P 2 1 / c , Z =4, a =11.3552(4), b =7.9893(3), c =15.7692(6) Å, β =101.013(1)°) represents a distorted variant of Rb 3 [B(SeO 4 ) 3 ]. The isolated pentamers in HK 4 [(B(SeO 4 ) 4 ]) (triclinic, P $\bar 1$ , Z =6, a =7.5303(1), b =7.5380(1), c =42.3659(4) Å, α =88.740(1), β =89.971(1), γ =89.971(1)°) were also found in K 5 [(B(SO 4 ) 4 ] and Na 5 [(B(SO 4 ) 4 ]. Compound (H 3 O)Na 6 [B(SeO 4 ) 4 ](SeO 4 ) (tetragonal, I $\bar 4$ , a =9.9796(1), c =18.2614(2) Å) is a super structure of the borophosphates Sr 6 [B(PO 4 ) 4 ](PO 4 ) and Pb 6 [B(PO 4 ) 4 ](PO 4 ). Because the tetrahedra are only connected through apices, there are topological analogies to silicates. Therefore, boroselenates may have a similar variability of crystal structures, such as borosulfates and borophosphates.