NaAsO2 reacts with Mo(CO)(6) in refluxing methanol or ethanol to yield [Et(4)N](2)[(OC)(5)MoAsMo3(CO)(9)(mu(3)-OR)(3)-Mo(CO)(3)] (R = Me, [Et(4)N](2)[Ia]; R = Et, [Et(4)N](2)[Ib]). The compound [Et(4)N](2)[Ia] crystallizes as the thf solvate, [Et(4)N](2)[Ia]. 0.6thf, in space group P2(1)/n, with a = 12.0420(46) Angstrom, b = 25.7455(81) Angstrom, c = 18.1991(56) Angstrom, beta = 94.277(28)degrees, and V = 5626(5.9) Angstrom(3), and was refined on F-2 to wR2 = 0.1802 and a conventional R1 = 0.0836. The anion [Ia](2-) is built around a tetrahedral AsMo3 core. A Mo(CO)(3) fragment is bonded to the Mo-3 base via three triply bridging methoxy ligands. The structure is completed by a Mo(CO)(5) fragment which is bonded to the lone pair of the As. The compound does not obey the usual electron-counting rules, and extended Huckel calculations on an indealized model with C-3v symmetry showed that the extra electron pair is located in an at orbital, equally delocalized over the three molybdenum atoms of the Mo-3 base.