Preparation, Structure, and Properties of the Arsenic-Containing Corner-Shared Double Cube [Mo6AsS8(H2O)18]8+: Metal−Metal Bonding and a Classification of Different Cluster Types

The trinuclear Mo IV 3 incomplete cuboidal cluster [Mo 3 S 4 (H 2 O) 9 ] 4+ in 2 M HCl reacts rapidly (green to intense blue-green color change) with AsO 2 - or HAsO 4 2- in the presence of H 3 PO 2 as reductant to give the new air-sensitive corner-shared double cube [Mo 6 AsS 8 (H 2 O) 18 ] 8+ . Purification by Dowex 50W-X2 cation-exchange chromatography requires 4 M H(pts) (pts = p -toluenesulfonic acid) for elution, consistent with a high charge. The X-ray structure has been determined for crystals obtained from 4 M H(pts) solution, full formula [Mo 6 AsS 8 (H 2 O) 18 ](pts) 8 ·28H 2 O. The Mo 6 AsS 8 core structure was confirmed from ICP elemental analysis for Mo, As, and S (6:1:8) and the 8+ charge from the 3:1 stoichiometries with [Co(dipic) 2 ] - and [Fe(H 2 O) 6 ] 3+ as oxidant. Rate constants for the oxidation of [Mo 6 AsS 8 (H 2 O) 18 ] 8+ with [Co(dipic) 2 ] - have been determined and indicate strong reduction properties as compared to other heteroatom clusters. The existence of short (bonding) Mo−Mo separations, 2.716 Å average, and long (nonbonding) Mo−As separations at 3.554 A, together with similar features for other heteroatom clusters, has led to a classification of the clusters into different categories. The nine 8+ double cubes, most of which contain a main group heteroatom M, are classified as (Mo 3 S 4 4+ ) 2 M 0, with retention of two metal−metal bonded Mo 3 S 4 4+ units. The single cubes, on the other hand, can be classified as Mo 3 S 4 4+ M 0, with M a transition metal, or Mo 3 S 4 4+ M x, with M a main group element ( x depends on group involved).