Structural Uniqueness Of The [Nb(As-5)(2)](5-) Cluster In The Zintl Phase Cs5nbas10

The structure of the novel Zintl phase, Cs5NbAs10, is reported for the first time. This compound crystallizes in the monoclinic P2(1)/c space group (no. 14) with eight formula units per cell. The structure represents a unique atomic arrangement, constituting a new structure type with Wyckoff sequence e32. The most important structural element is the unprecedented [Nb(As-5)(2)](5-) cluster anion, formed by a Nb atom enclosed between two As-5 rings. These nonaromatic cyclic species, formally [As-5](5-), adopt an envelope conformation similar to that of cyclopentane. To date, it is only the second example of an [As-5](5-) ring with this conformation, reported in an inorganic solid-state compound. The bonding characteristics of the [Nb(As-5)(2)](5-) cluster and the [As-5](5-) rings are thoroughly investigated using first-principles methods and discussed. Electronic band structure calculations on Cs5NbAs10 suggest that this compound is a semiconductor with an estimated band gap of ca. 1.4 eV.