The syntheses and crystallographic characterization of the linear trinuclear complex [Mo3O4(TPP)(3)]ClO4 and the monomeric precursors [MoO(TPP)(OClO3)] and [MoO(TPP)(OH2)]ClO4 an described. The crystallographic analyses of [MoO(TPP)(OClO3)] and [MoO(TPP)(OH2)]ClO4 show that these similar complexes are typical oxomolybdenum(V)-porphyrin complexes whose only significant structural differences are related to their respective axial ligands. [Mo3O4(TPP)(3)]ClO4 is obtained by the controlled hydrolysis of [MoO(TPP)(OClO3)] or by the direct reaction of [MoO(TPP)(OClO3)] with {[MoO(TPP)](2)O}. This hydrolysis product contains the novel [Mo3O4(TPP)(3)](+) ion, which has an unprecedented linear [O=Mo-O-Mo-O-Mo=O](7+) unit and three nearly parallel 24-atom porphyrin mean planes. The central porphyrin is found to be rotated 29.3 degrees and 29.7 degrees with respect to the two terminal porphyrin ligands, which are nearly eclipsed (twist angle 0.44 degrees). The formation of [Mo3O4(TPP)(3)]ClO4 instead of the mu-hydroxo-bridged binuclear complexes obtained from the hydrolysis of analogous mononuclear metalloporphyrin complexes is rationalized on the basis of the greater pi-acceptor ability of Mo(V). Crystal data for [MoO(TPP)(OClO3). 0.9CH(2)Cl(2): a = 11.437(2) Angstrom, b = 13.567(6) Angstrom, c = 13.764(4) Angstrom, alpha = 89.15(1)degrees, beta = 108.86(1)degrees, gamma = 103.96(2)degrees, triclinic, space group P (1) over bar, Z = 2, T = 293(2) K. Crystal data for [MoO(TPP)(OH2)ClO4. 1.5C(7)H(8): a = 13.2138(7) Angstrom, b = 13.5959(12) Angstrom, c = 14.6676(11) Angstrom, alpha = 74.292(8)degrees, beta = 69.114(8)degrees, gamma = 69.030(5)degrees, triclinic, space group P (1) over bar, Z = 2, T = 127(2) K. Crystal data for [Mo3O4(TPP)(3)]ClO4. C6H14. 3.4CH(2)Cl(2). 4H(2)O: a = 26.428(6) Angstrom, b = 18.683(6) Angstrom, c = 28.697(4) Angstrom, beta = 113.98(2)degrees, monoclinic, space group P2(1)/c, Z = 4, T = 127(2) K.