Organometallic nitrosyl complexes

A series of comprehensive reviews7,9-11 documents the historical growth of nitrosyl chemistry and also shows the recent expansion that has taken place both in the more traditional areas and in the comparatively new organometallic branch12 of this subject. In this section it is our intention to discuss the structural aspects of nitrosyl coordination, to provide an account of the methods available for the preparation of nitrosyl complexes and to survey the reactions that occur at the nitrosyl group. 

Optically active organometallic complexes have been used to study stereochemical reactions. Substituted cobalt nitrosyl complexes are interesting chiral see Chiral) complexes because they exhibit tetrahedral structures, whereas most optically active organometallic complexes are half-sandwich structmes with octahedral geometries. Diastereomeric cobalt complexes of the type Co(CO)(NO)(L)( L) (L = phosphite or phosphane L = optically active phosphane or isocyanide) have been synthesized from (4) via substitution (Scheme 6). ... 

Few successful reactions are known by which an organometallic nitrosyl product has been formed from the condensation of mononuclear metal nitrosyl complexes. The unique cluster (Tj -C5H5)3Mn3(NO)4 (39), which contains the only i3-NO known (7) (40), is prepared using photochemical or thermal (41) decarbonylation of (Tj -C5H5)2Mn2(CO)2(NO)2 [Eq. (25)]. The use of the redox condensation of a nitrosyl carbonylmetallate with a neutral metal carbonyl cluster to produce a nitrosyl cluster has... 

The cubane-like cluster [ Fe(/X3-S)(NO) 4] (68) is not formally an organometallic compound, but the structural changes observed when it is reduced, and the MO description of its bonding, are relevant to the cyclopentadienyl complexes described above. The nitrosyl complex contains 60 valence electrons, 8 less than [ Fe()U3-S)Cp 4], and is reduced in two consecutive one-electron steps to an isolable monoanion [68] , and to a dianion [68] . 

Keeping our target of homogeneous catalysis in mind, we adopted a broad definition of organometallic complexes and included compounds without metal-carbon bonds (e. g., metal-phosphine and metal-nitrosyl complexes) as far as they retain the structural and reactivity features of typical organometallic compounds. Recent developments, e. g., the substitution of phosphine by carbene ligands (cf Section 3.1.1.1), support the validity of this view. 

The 18-electron rule is not obeyed as consistently by these types of organometallic compounds as by the carbonyl and nitrosyl complexes and their derivatives. For example, in addition to ferrocene. M(i7 -C5H5)2 compounds are known for most of the other elements of the first transition series (M = V, Cr. Mn. Co, Ni) and these cannot obey the 18-electron rule. However, only ferrocene shows exceptional thermal stability (stable to 500 °C) and is not oxidized by air. Furthermore, cobaltocene, a 19-electron species, is readily oxidized to the 18-electron cobaltocenium ion, [Co(t7 -CjH5)2J, which reflects much of the thermal stability of ferrocene. Mixed cyclopentadienyl carbonyl complexes are common [(- j -C3H5)V(CO)J, [(tj -CjHj)-Cr(CO)3J2, [(77 -CjH5)Mn(CO)3], [(77- -C3H5)Fe(CO)2)2. [(t, -CjH5)Co(CO)2],... 

Apart from their natural biological functions various metal nitrosyls (including organometallic dinitrosyl complexes of Cr and Mo) proved to be very convenient carriers supplying nitric oxide to various biological targets and are presently widely introduced into the therapy as vasodilators and in cancer treatment [350-352], see the chapter by Morris et al. in this volume of S B. 

Dudle B, Rajesh K, Blacque O, Berke H (2011) Rhcaiium nitrosyl complexes bearing large-bite-angle diphosphines. Organometallics 30 2986-2992... 

Choualeb A, Maccaroni E, Blacque O, Schmalle HW, Berke H (2008) Rhenium nitrosyl complexes for hydrogenations and hydrosilylations. Organometallics 27 3474—3481... 

Cboualeb A, Lougb AJ, Gusev DG (2007) Hydridic rhenium nitrosyl complexes with pmcer-type PNP ligands. Organometallics 26 3509-3515... 

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