A series of molybdenum(IV) complexes of the type trans-Mo(Q)(Q’)(PP)(2) has been prepared where Q and Q’ are chalcogen ligands (O, S, Se, Te) and PP is either cis-1,2-bis(diphenylphosphino)ethylene (dppee) or 1,2-bis(diphenylphosphino)ethane (dppe). X-ray crystallographic studies were carried out to investigate how the Mo-Q distance is influenced by the mutually competitive d(pi)-p(pi) interactions between the p(x) and p(y) orbitals of the chalcogen ligands and the d(xz) and d(yz) orbitals of the molybdenum center. trans-Mo(O)(2)(dppee)(2) (1) has been prepared by the hydrolysis and deprotonation reaction of [Mo(O)(Cl)(dppee)(2)]Cl with NaOH in methanol. The compounds where Q represents the heavier chalcogens (S, Se, Te) have been prepared by the reaction between trans-Mo(N-2)(2)(PP)(2) and a chalcogen source : trans-Mo(S)(2)(dppee)(2) (2), BzS(3)Bz (dibenzyl trisulfide); trans-Mo(Se)2(dppee)2 (3), elemental Se; trans-Mo(Te)(2)(dppee)(2) (4), TePEt3 (Et = ethyl); trans-Mo(S)(2)(dppe)(2) (5), BzS(3)Bz; trans-Mo(Se)(2)(dppe)(2) (6), Se. trans-Mo(O)(S)(dppee)(2) (7) was obtained from the reaction of MoCl3-(THF)(3) (THF = tetrahydrofuran), dppee, and NaHS in a mixture of THF and methanol. The attempted preparation of 7 by the reaction of SO2 and trans-Mo(N-2)(2)(dppee)(2) yielded Mo(SO2)(2)(dppee)(2) (8). 1 crystallizes in the monoclinic space group P2(1)/c (No. 14, Z = 2) with a = 11.1340(15) Angstrom, b = 18.435(2) Angstrom, c = 12.515(2) Angstrom, and beta = 110.999(9)degrees; 2 crystallizes in the triclinic space group P (1) over bar (No. 2, Z = 1) with a = 10.102(4) Angstrom, b = 10.722(4) (4) Angstrom, c = 12.195(3) Angstrom, alpha = 100.95(3)degrees, beta = 95.04(4)degrees, and gamma = 117.81(2)degrees; 3 crystallizes in the monoclinic space group P2(1)/c (No. 14, Z = 2) with a = 11.186(5) Angstrom, b = 18.005(8) Angstrom, c = 12.761(9) Angstrom, and beta = 110.35(4)degrees; 4 crystallizes in the triclinic space group P (1) over bar (No. 2, Z = 2) with a = 12.681(4) Angstrom, b = 19.280(5) Angstrom, c 10.454(3) Angstrom, alpha = 104.60(2)degrees, beta = 111.61(2)degrees, and gamma = 75.12(2)degrees; 5 crystallizes in the monoclinic space group C2/c (No. 15, Z = 8) with a = 49.515(7) Angstrom, b = 10.9286(12) Angstrom, c = 18.203(3) Angstrom, and beta = 98.306(12)degrees; 6 crystallizes in the monoclinic space group C2/c (No. 15, Z = 8) with a = 49.566(9) Angstrom, b = 10.9765(15) Angstrom, c = 18.282(3) Angstrom, and beta = 98.541(13)degrees; 7 crystallizes in the triclinic space group P (1) over bar (No. 2, Z = 1) with a = 10.040(1) Angstrom, b = 10.563(1) Angstrom, c = 12.162(2) Angstrom, alpha = 75.30(1)degrees, beta = 85.93(1)degrees, and gamma = 63.21(1)degrees; 8 crystallizes in the monoclinic space group C2/c (No. 15, Z = 4) with a = 21.534(6) Angstrom, b = 12.4271(13) Angstrom, c = 19.550(5) Angstrom, and beta = 118.480(14)degrees. The UV/vis and the P-31{H-1} NMR data for compounds 1-7 are also reported and discussed.