Treatment of [Me(4)N](2)[PhP(CH2CH2S)(2)] with [Mo(CO)(3)(NCMe)(3)] affords the reactive intermediate [Me(4)N](2)[Mo(CO)(3){PhP(CH2CH2S)(2)}] (1), which undergoes oxidation to afford [Mo{PhP(CH2CH2S)(2)}(2)] (2). Reaction of 1 with a variety of dichloroalkanes produces [Mo(CO)(3){c-PhP(CH2CH2S)(2)X}] (X = CH2CH2, CH2CH2CH2, CH(2)CHMe or CH2CH(OH)CH2). The structure of [Mo(CO)(3){c-PhP(CH2CH2S)(2)CH2CH2}] (3) has been established by X-ray crystallography and consists of a Mo(CO)(3) fragment facially coordinated by the tridentate c-PhP(CH2CH2S)(2)CH2CH2 ligand. Reaction of 3 with bromine affords seven-coordinate [Mo(CO)(2){c-PhP(CH2CH2S)(2)CH2CH2}Br-2] (7), the X-ray crystal structure of which reveals a carbonyl-capped octahedral geometry. Treatment of 3 with sulfur results in loss of the Mo(CO)(3) fragment and is elation of c-PhPS(CH2CH2S)(2)CH2CH2 (8), the X-ray structure of which shows a nine-membered ring with the phosphorus center bearing phenyl and sulfide substituents. Reduction of 8 with sodium naphthalenide affords the parent ligand c-PhP(CH2CH2S)(2)CH2CH2. Crystal data : 2, C20H26MoP2S4, triclinic P (1) over bar, a = 8.105(3) Angstrom, b = 8.263(3) Angstrom, c = 17.663(4) Angstrom, alpha = 100.29(2)degrees, beta = 99.78(2)degrees, gamma = 98.81(2)degrees, Z = 2; 3, C15H17MoO3PS2, monoclinic P2(1)/n, a = 9.600(3) Angstrom, b = 15.594(5) Angstrom, c = 11.335(3) Angstrom, beta = 93.01(2)degrees, Z = 4; 7, C14H17Br2MoO2PS2, monoclinic P2(1)/c, a = 17.039(3) Angstrom, b = 8.686(2) Angstrom, c = 12.466(3) Angstrom, beta = 100.52(2)degrees, Z = 4; 8, C12H17PS3, monoclinic P2(1), a = 6.651(4) Angstrom, b = 7.313(2) Angstrom, c = 14.687(9) Angstrom, beta = 101.62(3)degrees, Z = 2.