Unexpected direct incorporation of NO in the Mo(IV) coordination sphere. X-ray crystal structure of (PPh 4 ) 4 [{Mo(CN) 5 (NO)} 2 (l-pz)] Æ 2C 2 H 5 OC 2 H 5 Dariusz Matoga a , Janusz Szklarzewicz a, * , John Fawcett b a Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krako ´ w, Poland b Department of Chemistry, University of Leicester, Leicester LE1 7RH, UK Received 18 February 2005; accepted 1 April 2005 Available online 20 June 2005 Abstract The preparation and characterization of a stable cyano {MoNO} 4 dimer with bridging pyrazine (pz), [{Mo(CN) 5 (NO)} 2 - (l-pz)] 4 , is described. The synthesis involves the direct incorporation of nitric oxide into the [Mo(CN) 4 O(pz)] 2 complex and unexpected oxo ligand release. The IR monitoring of the reaction in solution indicates its very complicated mechanism with NO association and nitro ligand formation steps. The isolated diamagnetic tetraphenylphosphonium salt of the dimer is characterized by means of elemental analysis, ESI-MS, cyclic voltammetry measurements as well as IR, EPR and UV–Vis spectroscopy. The molecular structure of (PPh 4 ) 4 [{Mo(CN) 5 (NO)} 2 (l-pz)] Æ 2C 2 H 5 OC 2 H 5 shows that the N–O bond [1.198(4) A ˚ ] in the nitrosyl ligand is longer than in gaseous nitric oxide and the Mo–N–O group is approximately linear [Mo–N–O angle is 178.2(3)°], which is expected for species with a low {MNO} electron number. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Molybdenum; Cyanides; X-ray diffraction; Structure; Nitric oxide; Complexes; Pyrazine (bridging ligand) 1. Introduction During the last two decades there has been a growing interest in nitrosyl metal complexes resulting to a large degree from the revealed new and unexpected role for nitric oxide as a key physiological regulator [1–8]. There are numerous examples of nitrosyl complexes of molyb- denum and tungsten, the majority of which contain the metal in a low oxidation state (II or 0) [8–23]. In this group, cyanonitrosyl complexes are very rare. Among the known entities of that type are the series of pentacy- anonitrosylmolybdates: [Mo(CN) 5 (NO)] 2/4 ions [24– 26], obtained generally in the reaction of MoO 4 2 with hydroxylamine in the presence of cyanide ions, as well as the paramagnetic [Mo(CN) 5 NO] 3 ion, being the oxi- dation product of [Mo(CN) 5 NO] 4 by hypobromite [25]. There are also known [Mo(CN) 3 (NO)(L) 2 ] com- plexes, prepared from [Mo(CN) 5 (NO)] 2 by the substi- tution of two CN  ligands with various pyridine derivatives, as well as tetracyanonitrosylmolybde- num(0), [Mo(CN) 4 (NO) 2 ] 2 and the series of [Mo(CN) 2 (NO) 2 (L) 2 ] Æ 2H 2 O compounds, obtained by the substitution of cyanides with L Lewis bases [8]. Moreover, the polymeric Prussian blue analogue, K 0.9 Mn II 1.05 ½Mo I ðCNÞ 5 ðNOÞ  5H 2 O  1.9MeOH, has been described, showing spontaneous magnetization with a Curie temperature of 39 K [27]. In the group of cyanonitrosyl complexes of molybdenum and tungsten, there is only one known example of a structurally char- acterized complex, neutral [Mo(CN)(HNO)(NO)(terpy)] (where terpy = 2,2 0 :6 0 ,2 00 -terpyridine), prepared from the 0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.04.024 * Corresponding author. Tel.: +48 12 6632231; fax: +48 12 6340515. E-mail address: szklarze@chemia.uj.edu.pl (J. Szklarzewicz). www.elsevier.com/locate/poly Polyhedron 24 (2005) 1533–1539