Treatment of the sulfur monocapped molybdenum trinuclear cluster (NH4)(2)[MO(3)S(13)] With excess trimethylphosphine affords the sulfur bicapped molybdenum trinuclear cluster complex [Mo-3(mu(3)-S)(2)(mu(2)-S)3(PMe(3))(6)] This compound crystallizes in the rhombohedral space group R $($) over bar$$ 3c with a = 11.112(1) Angstrom, c = 53.507(10) Angstrom, V = 5722(1) Angstrom, Z = 6, R = 0.043, and R(w) = 0.026. The Mo-3 triangle is equilateral with the Mo-Mo distance 2.714(1) Angstrom. Calculations by the DV-X alpha method on this compound show that the structural difference from an isoelectronic cluster anion [Mo3S2Cl9](3-) is caused by the reversal of the HOMO and LUMO. Calculated populations and a qualitative molecular orbital discussion reveal that this reversal is due to the pi antibonding interaction between the metal atoms and bridging ligands. Construction of qualitative MOs clarifies the effect of the M-L pi interaction on the MOs derived from metal t(2g), orbitals in the M(3)X(5)L(6)-type and M(3)X(4)L(9)-type clusters where metal atoms are in the octahedral coordination.