A series of molybdenum and tungsten nitrido, [M( N)(X)(diphos)(2)], and imido complexes, [M(NH)(X)(diphos)(2))] Y, ( M) Mo, W) with diphosphine coligands (diphos) dppe/depe), various trans ligands (X = N-3(-), Cl-, NCCH3) and different counterions (Y- = Cl-, BPh4-) is investigated. These compounds are studied by infrared and Raman spectroscopies; they are also studied with isotope-substitution and optical-absorption, as well as emission, spectroscopies. In the nitrido complexes with trans-azido and - chloro coligands, the metal - N stretch is found at about 980 cm(-1); upon protonation, it is lowered to about 920 cm(-1). The (1)A(1) --> E-1 ( n --> pi*) electronic transition is observed for [Mo( N)(N-3)( depe)(2)] at 398 nm and shows a progression in the metal - N stretch of 810 cm(-1). The corresponding E-3 --> (1)A (pi* --> n) emission band is observed at 542 nm, exhibiting a progression in the metal - N stretch of 980 cm(-1). In the imido system [Mo(NH)(N-3)(depe)(2)] BPh4, the n --> pi* transition is shifted to lower energy ( 518 nm) and markedly decreases in intensity. In the trans-nitrile complex [Mo(N)(NCCH3)(dppe)(2)] BPh4, the metal N( nitrido) stretching frequency increases to 1016 cm(-1). The n --> pi* transition now is found at 450 nm, shifting to 525 nm upon protonation. Most importantly, the reduction of this nitrido trans-nitrile complex is drastically facilitated compared to its counterparts with anionic trans-ligands (E-p(red) = - 1.5 V vs Fc(+)/Fc). On the other hand, the basicity of the nitrido group is decreased (pK(a) {[Mo(NH)(NCCH3)(dppe)(2)](BPh4)(2)} = 5). The implications of these findings with respect to the Chatt cycle are discussed.