Electronic absorption and emission spectra are reported for salts of two oxomolybdenum(IV) cations, [MoOCl(CN-t-Bu)(4)](+) and [MoOCl(Ph2PCH2CH2PPh2)(2)](+), and for the new Mo(lV) complex [trans-Mo(OCH3)(2)(CN-t-Bu)(4)](2+). All three ions show absorption bands (lambda(max,abs) 550-570 nm; epsilon 45-120 M-1 cm(-1)) attributable to the (1)A(1)[(d(xy))(2)] --> E-1[(d(xy))(1)(d(xz,yz))(1)] (C-4v) transition, and the last two show weak shoulders in the 700-750 nm range due to the analogous spin-forbidden ((1)A(1) --> E-3) transition. Phosphorescence (lambda(max,em) 850-960 nm) occurs in the solid state for all three compounds at both room temperature and 77 K, and for [MoOCl(CN-t-Bu)(4)](+) in CH2Cl2 at room temperature. These are the first phosphorescences recorded for molybdenum(IV) complexes. [MoOCl(CN-t-Bu)(4)](BPh4) precipitates quickly if NaBPh4 is added to the Mo(IV) solution prepared from MoCl5 and tert-butyl isocyanide in CH3OH. However, if NaPF6 is used instead, [trans-Mo(OCH3)(2)(CN-t-Bu)(4)](PF6)(2) (formed by reaction of [MoOCl(CN-t-Bu)(4)](+) with methanol) crystallizes over a period of ca. 24 h. The crystal structure of [trans-Mo(OCH3)(2)(CN-t-Bu)(4)](PF6)(2) has been determined : C22H42F12MoN4O2P2, monoclinic; space group P2(1)/c; a = 9.1538(8) Angstrom, b = 15.709(2) Angstrom, c = 13.456(2) Angstrom; beta = 103.31(1)degrees; Z = 2; R(F) = 0.063, R-w(F) = 0.056 for 2719 reflections with I > sigma(I).