The title compound [N(CH 3) 4] 4 [H 5PMo 5V 9O 42 ] ·3.5H 2O was synthesized by hydrothermal method and its structure was determined with single crystal X ray analysis. The cluster anion is a bicapped Keggin polyanion, two vanadium atoms are distributed at two capping metal atom positions. The crystal is monoclinic, space group C2/c with a=1.284 8(4) nm, b=2.366 4(4) nm, c=2.113 6(4) nm, β= 90.69(2)° , V=6.425(4) nm 3, Z=4, D c=2.07 g\5cm -3 , R=0.066, R w=0.069, S=1.33, \{(Δ/σ) max =\}0 04, \{(Δρ) max =\}91 e·nm -3 .
