Chromium nitrosyls, coordination

Compared to the more extensively studied metal clusters of NO, there are only a handful of examples of monomeric or dimeric nitrosyl complexes undergoing N-O bond dissociation. Cummins and coworkers [117] reported novel in-termolecular NO reductive cleavage reactions of chromium nitrosyl complexes utilizing a highly reactive low-coordinate V htriaryl complex (THF)V(Mes)3 (Mes = 2,4,6-C6H2Me3) as the 0-atom acceptor (Scheme 2.4). 

Monomeric species M OR-tert)x have been characterized for titanium, vanadium, chromium, zirconium, and hafnium (x = 4) and for niobium and tantalum (x == 5). With chromium it was found that limiting Cr(III) to coordination number 4 in the dimeric Cr2(OBu )e caused instability and a remarkable facility toward valency disproportionation or oxidation to the stable quadricovalent Cr(OBu )4 (8, 9). In contrast, molybdenum formed a stable dimeric tri-tert-butoxide (Bu O)3Mo=Mo-(OBu )3 which is diamagnetic and presumably bound by a metal-metal triple bond (10, II). Yet another interesting feature of chromium is the synthesis of a stable diamagnetic nitrosyl Cr(NO) (OBu )3 in which the nitric oxide is believed to act as a three-electron donor with formation of a four-coordinated low spin chromium (II) compound (12). The insta-bihty of Cr2(OBu )e and the stability of both Cr(NO) (OBu )3 and Cr(OBu )4 must result from the steric effects of the tertiary butoxo groups since the less bulky normal alkoxo groups form very stable polymeric [Cr(OR)3]a. compounds in which the Cr(III) has its usual coordination number of 6 (octahedral). 

The NO complexes of chromium that have been reported to date exist in the nitrosyl forms. In coordination complexes in particular, several compounds of configuration Cr(NO) have been isolated these have been reviewed recently (80,81). In a 2005 review. Ford and coworkers noted that photolysis of some Fe, Mn, and Cr nitrosyl complexes often leads to the dissociation of the nitrosyl ligands (82). For example, photolysis of the [(H20)5Cr(N0)] complex under anaerobic conditions, and in the... 

Low oxidation state metal ions are used in selective reduction of coordinated ligands. In particular, Armorl2i has shown that chromium(II) chloride reduces the nitrosyl ligand in [Ru(NH3)5NO] +, Eq. 9.33 ... 

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