Synthetic procedures and preliminary chemical studies are described for four new types of Group VIa nitrosyl-arenediazo complex: (η-C 5H 5)M(NO)(N 2Ar)Cl (VII), (HBpz 3)M(NO)(N 2Ar)Cl (VIII). [(HCpz 3)M(NO)(N 2Ar)Cl] + (IX) and [(η-C 5H 5)M(NO)(N 2Ar)(PPh 3)] + (XIII). These complexes complete the homologous series containing the isoelectronic and quasi-isostructural fragments [M(NO) 2] 2+, [M(NO)(N 2Ar)] 2+ and [M(N 2Ar) 2] 2+ (M = Group VIa metal). IR spectroscopic studies of VII and VIII suggest that the hydrotris-(1-pyrazolyl)borato ligand in these complexes is a better π-acceptor than cyclopentadienide. Two of the new complexes have been characterised by X-ray crystallographic methods. Crystals of (HBpz 3)Mo(NO)(N 2C 6H 4F- p)Cl (VIIIb) are triclinic, space group P 1 with four molecules in a unit cell of dimensions a = 13.053(3), b = 18.275(3), c = 8.954(2) Å, α = 97.18(2), β = 106.50(2), γ = 88.53(2)°. Crystals of [(η-C 5H 5)Mo(NO)(N 2C 6H 4F- p)(PPh 3)]PF 6, [(XIIIc)]PF 6, are also triclinic, space group P 1 , with two formula units in a unit cell of dimensions a = 11.526(1), b = 13.875(2), c = 10.873(2) Å, α = 108.37(2), β = 105.82(10, γ = 103.00(1)°. Both structures were determined by the heavy-atom method and refined to final R values of 0.030 (for 3812 observed reflections, VIIb) and 0.028 (for 3560 observed reflections [(XIIIc)]PF 6). There are two independent molecules of VIIIb in the asymmetric unit. Principal bond lengths are: VIIIb: MoN(nitrosyl) 1.767(4) and 1.789(4), MoN(arenediazo) 1.857(3) and 1.877(3), MoN(Pz) 2.158–2.200(4), MoCl 2.424(1) and 2.428(1) Å; [(XIIIc)]PF 6: MoN(nitrosyl) 1.802(3), MoN(arenediazo) 1.876(3), MoP 2.487(1), MoC(C 5H 5) 2.318–2.371(4) Å.