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Graphical abstract Highlights • Acenaphthenequinonediimine Mo(II) complexes were intercalated in Layered Double Hydroxides. • The complexes and materials were active in epoxidation of several olefins. • The materials competed effectively with the complexes in the epoxidation of most substrates. Abstract The two layered double hydroxides ZnAl-LDH and MgAl-LDH were functionalized with bis(4-HOOC-phenyl)-acenaphthenequinonediimine) (H 2 BIAN), leading to the intercalation of its dianion, which in a second step reacted with [Mo(CO) 3 X 2 (NCMe) 2 ] (X = I, Br), affording four new materials. These materials and the two complexes [Mo(CO) 3 X 2 (H 2 BIAN) 2 ] (X = I, Br) were tested in the olefin epoxidation reaction with substrates cis -cyclooctene, styrene, 1-octene, trans -hex-3-en-1-ol, and R- (+)-limonene, using tert -butylhydroperoxide (tbhp) as oxidant. The new catalysts were particularly good for cis -cyclooctene and styrene (100% conversions) and at least one heterogeneous catalyst was comparable to the homogeneous ones in the epoxidation of 1-octene and trans -hex-3-en-1-ol. The homogeneous catalysts were the best to oxidize R- (+)-limonene (higher conversions). [ABSTRACT FROM AUTHOR]

A series of dimeric N -hetercycle oxomolybdenum(V) complexes cis -Mo 2 O 4 (SO 4 )(phen) 2 ·2H 2 O ( 1 ), cis -Mo 2 O 4 (SO 4 )(bpy) 2 ·4H 2 O ( 2 ), cis -Mo 2 O 4 Cl 2 (bpy) 2 ( 3 ) and cis -Mo 2 O 4 (SO 4 )(im) 4 ( 4 ) (phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine, im = imidazole) have been synthesized and characterized by the elemental analyses, IR, UV–vis, molecular fluorescence spectra, X-ray structural analyses and theoretical calculations. The two N -hetercycle ligands in 1 or 2 exhibit an approximate plane owing to SO 4 2− steric hindrance with strong interactions (2.215 av and 2.186 av  Å), which are new discrete dinuclear oxomolybdenum(V) complexes. 1 – 3 have been synthesized in acidic media under hydrothermal conditions and the transformations of oxomolybdenum(V) 2 – 3 and 1 to cis -Mo 2 O 4 Cl 2 (phen) 2 ( 5 ) are also outlined respectively. Moreover, the catalytic performances of 1 – 5 were tested by the degradation of methyl orange. The conjugated N -hetercycle ligands have obvious influences on their catalytic performances. [ABSTRACT FROM AUTHOR]

The Mo(II) complexes [MoX 2 (CO) 3 (BDC)] (X = I, Br; H 2 BDC = 2,2′-bipyridine-5,5′-dicarboxylic acid) were immobilized by intercalation in two layered double hydroxides (LDH), containing Zn/Al (synthesized) and Mg/Al (commercial), as shown by infrared and 13 C solid state NMR spectroscopies, and X-ray powder diffraction. Their catalytic activity was studied in olefin oxidation, using as oxidant tert -butylhydroperoxide (tbhp), and was compared with the activity of the related [MoX 2 (CO) 3 (bpy)] complexes (bpy = 2,2′-bipyridine). cis -Cyclooctene, styrene, 1-octene, trans -hex-3-en-1-ol, and R -(+)-limonene were the substrates tested. All the catalyst showed in average a high selectivity toward the epoxide of each substrate (above 90%, except for styrene), with variable conversions, but none of the heterogeneous catalysts proved to be more active than the homogeneous ones. The same or higher activity in a second catalytic run, as observed in most reactions, suggests a good recycling capability. [ABSTRACT FROM AUTHOR]

Three new molybdenum complexes have been synthesized from one-pot reactions between Mo(CO) 3 (NCMe) 3 and pyridine-2-thiol (pySH) in the presence of different P-donors. Reaction with P(OMe) 3 in MeCN at ca. 55 °C gives Mo(CO) 2 [P(OMe) 3 ](κ 2 -pyS) 2 ( 1 ) and Mo(CO) 4 [P(OMe) 3 ] 2 ( 2 ). A similar reaction involving bis(diphenylphosphino)methane (dppm) furnishes the dinuclear species Mo 2 (CO) 4 (μ,κ 2 -pyS) 2 (κ 2 -dppm) ( 3 ) together with the mononuclear complexes Mo(CO)(κ 2 -pyS) 2 (κ 2 -dppm) ( 4 ) and Mo(CO) 4 (κ 2 -dppm) ( 5 ). With bis(diphenylphosphino)ethane (dppe) only the mononuclear complexes Mo(CO)(κ 2 -pyS) 2 (κ 2 -dppe) ( 6 ), Mo(CO) 4 (κ 2 -dppe) ( 7 ), and Mo(CO) 3 (κ 1 -dppe)(κ 2 -dppe) ( 8 ) resulted. Complexes 1 , 3 , and 6 have been characterized by single-crystal X-ray diffraction analyses and the bonding in 1 and 3 has been evaluated by density functional theory (DFT) calculations, optimized structures being discussed relative to the observed solid-state structures. [ABSTRACT FROM AUTHOR]

Molybdenum complexes of type [MoO 2 L] n (L = tridentate ONO Schiff base ligands based on the salicylidene-2-aminophenolato structure) have been synthesized and used as catalysts for the solvent-free epoxidation of olefins. The effect of donor (diethylamino) and/or acceptor (nitro) substituents has been studied. [ABSTRACT FROM AUTHOR]

Four dioxomolybdenum(VI) complexes were synthesized by reacting [MoO2(acac)2] with thiosemicarbazone ligands derived from 5-chloro-2-hydroxybenzaldehyde (H2L1), 2-hydroxy-5-methylbenzaldehyde (H2L2), 3-tert-butyl-2-hydroxybenzaldehyde (H2L3), or 2,3-dihydroxybenzaldehyde (H2L4). In all the complexes, the ligands were coordinated to molybdenum as tridentate ONS donors. X-ray crystallography showed that the distorted octahedral coordination of molybdenum atom is completed by methanol molecule (D) as in [MoO2(L1D)], [MoO2(L2D)], and [MoO2(L3D)], or by an ethanol molecule as in [MoO2(L4D)]. The molecular structures of H2L2, H2L3, and all synthesized complexes were determined via single crystal X-ray crystallography. The binding properties of the ligand and the complexes with calf thymus DNA were analyzed by UV, fluorescence titration, and viscosity measurements. Gel electrophoresis shows that all complexes can cleave the pBR322 plasmid DNA. The cytotoxic properties of the complexes were studied against human colorectal cell lines. All complexes showed strong antiproliferative activities in the relative order [MoO2(L3D)] > [MoO2(L1D)] ~ [MoO2(L2D)] > [MoO2(L4D)] with IC50 values of 3.2, 4.5, 4.6, and 6.4 µM, respectively. The complexes exhibited greater pronounced activity than the standard reference drug, 5-fluorouracil, did (IC50 = 7.3 µM). These studies demonstrate the potential application of dioxomolybdenum(VI) complexes in chemotherapy. [ABSTRACT FROM AUTHOR]

Highlights: [•] Transformations among different types of citrato molybdates were studied in solid and solution. [•] Several experimental conditions emerged as the controlling factor of structural variation. [•] Each citrate acts as a tridentate ligand while a β-carboxy group is free. [Copyright &y& Elsevier]

Highlights: [•] We prepared novel molybdenum–tricarbonyl complexes bearing macrocyclic ligands. [•] Molecular structure of complex has intra- and intermolecular hydrogen bond. [•] We compare CO stretching vibration of several carbonyl complexes by FT-IR. [•] Hydrogen bond interaction controls the environment around metal center. [Copyright &y& Elsevier]

Abstract: Novel molybdenum and tungsten complexes with tridentate bis(phenolate) ligands containing [O,X,O] donor atoms (X=S or Se), [MoO2LS] (1), [MoO2LSe] (2), [WO2LS] (3) and [WO2LSe] (4), were synthesized and characterized as functional models for molybdenum- and tungsten-dependent oxidoreductases. The catalytic oxo-transfer properties of 1–4 were investigated by oxo-transfer reactions from nitrate to PPh3 with a ratio of complex (1–4):PPh3:(Bu4N)(NO3 −)=1:10:20 and from DMSO to PPh3 with varied catalyst:PPh3 ratios. For the oxygen atom transfer reaction from nitrate to PPh3, the molybdenum compounds are catalytically more active than their tungsten analogues. Whereas for the oxygen atom transfer reaction from DMSO to PPh3, the tungsten compounds exhibit more efficient catalytic behavior than the molybdenum analogues. In all catalytic reactions, sulfur containing compounds have a higher catalytic ability than their selenium analogues. In comparison, all four compounds catalyze the oxygen atom transfer reaction from DMSO to PPh3 much more effectively than the oxidation of PPh3 with NO3 −. [Copyright &y& Elsevier]

Abstract: The molecular structures of the phosphinoarylthiolato molybdenum and iron complexes [Mo{(SC6H4-2-PPh2)-κ2 S,P}2(CO)2] (1) and [Fe{(SC6H4-2-PPh2)-κ2 S,P}2(CO)2] (2) are reported. They both have a central hexacoordinate metal(II) cation, but compound 1 has a trigonal-prismatic geometry with both CO and both thiolato groups in cis arrangement, while iron(II) complex 2 displays a pseudo-octahedral structure with both CO, both phosphino and both thiolato groups in trans arrangement. This is in a good agreement with DFT calculations, which showed that the different coordination geometry of molybdenum and iron in complexes 1 and 2 is a consequence of the number of d electrons (electronic factor) combined with the geometrical factors. [Copyright &y& Elsevier]

Abstract: The synthesis of two Mo(VI) tetradentate Schiff base complexes derived from 2,2′-dimethylpropylenediamine and aromatic aldehydes, (MoO2{hnaphnptn} (1) and MoO2{salnptn(3-OMe)2} (2)) is reported. Full characterization of these complexes was accomplished with elemental analyses, spectroscopic studies (1H NMR, IR, and UV–Vis) and X-ray structure analysis. X-ray crystallography studies reveal that these complexes adopt a distorted octahedral six-coordinate configuration formed by tetradentate Schiff base ligand and two binding oxygen atoms. Catalytic performance of the prepared molybdenum complexes for oxidation of different olefins with tert-butyl hydroperoxide was evaluated. These complexes were found to be an efficient and selective catalyst for the homogeneous oxidation of various olefins. MoO2{salnptn(3-OMe)2} with a methoxy groups on the salicylidene ring of the ligand promote the effectiveness of the catalyst. [Copyright &y& Elsevier]

Abstract: A series of heterobimetallic polymeric complexes of manganese, cobalt, zinc, cadmium and nickel, [M(Mo2O5L2)(MeOH)2(H2O)2] n ·nH2O {M=Mn (2), n =1, Co (3), n =0, Zn (4), n =1 and Cd (5), n =1} and [Ni(Mo2O5L2)(MeOH)(H2O)3] n ·2H2O·MeOH (6) have been synthesized form the reaction of [{Na4(H2O)4(μ-H2O)2}⊂(Mo2O5L2)2] (1) {LH2 =2-(3,5-di-tert-butyl-2-hydroxybenzylamino)acetic acid} with the corresponding metal salts. The complexes have been structurally characterized. The Complexes, 3 and 6 undergo thermal decomposition to afford mixed oxides of the type, MMoO4·MoO3 {M=Co or Ni}. [Copyright &y& Elsevier]

Abstract: The molybdenum hydride complexes Mo(PMe3)5H2 and Mo(PMe3)4H4 are capable of cleaving the C–S bonds of thiophene, benzothiophene and dibenzothiophene. For example, Mo(PMe3)5H2 reacts with thiophene to give the η5-thiophene and butadiene–thiolate complexes, (η5-C4H4S)Mo(PMe3)3 and (η5-C4H5S)Mo(PMe3)2(η2-CH2PMe2). These complexes are also obtained from the reaction between Mo(PMe3)4H4 and thiophene under photochemical conditions, whereas at elevated temperatures thiophene is desulfurized to liberate but-1-ene. Similarly, Mo(PMe3)4H4 desulfurizes benzothiophene at elevated temperatures to liberate ethylbenzene, while the arylthiolate complex Mo(PMe3)4(SC6H4Et)H3 is obtained photochemically. Furthermore, Mo(PMe3)4H4 cleaves the C–S bond of dibenzothiophene to give [η6,κ1-C6H5C6H4S]Mo(PMe3)2H. [Copyright &y& Elsevier]

Abstract: The unusual 18e seven-coordinate Mo(II) complex [Mo(NO)(H2NO-κ 2 N,O)(TpMe2)I] (1; [TpMe2]− is hydrotris(3,5-dimethylpyrazol-1-yl)borate) has been synthesised and characterised by IR, 1H NMR and ESI-MS spectroscopies and by a single crystal X-ray diffraction study. The complex has a distorted pentagonal bipyramid structure with equatorial κ 2-NH2O− ligand (d N–O =1.387Å, d Mo–N and d Mo–O equal 2.049 and 2.092Å, respectively). In the solid state 1 exists as a dimer (the point group C i) due to the formation of two NH⋯O hydrogen bonds (d N–H⋯O =2.064 Å) between the adjacent NH2O− ligands, whilst in solution at/or above RT it resolves itself giving a monomer, which readily isomerises to more thermodynamically stable diastereoisomer. [Copyright &y& Elsevier]

Abstract: Reactions of the molybdenum and tungsten precursors [MO2S2]2− with equimolar amounts of benzenedithiol in acetonitrile give the title compounds [M2O2(μ-S)2(bdt)2]2− with M=Mo, W and bdt=benzene-dithiolate. In case a tungsten to ligand ratio of 1:2 is used the dimer forms as well but only as a minor species whereas the monomer [WO(bdt)2]2− is the main product. In both dimeric compounds the syn-isomers are formed referring to the position of the apical oxo ligands with respect to the M2S2 plane. For the molybdenum compound this contrasts with a published crystal structure of the anti-isomer. Both complexes give highly symmetric isomorphous crystals but still show subtle differences in their bond lengths and angles around the central metal. The X-ray crystal structures of both are analyzed in detail and compared with each other and with the isomeric molybdenum compound. Differences and similarities between tungsten and both isomers of molybdenum complexes are shown to be more influenced by the conformation than by the central metal and a reason for the formation of syn- and anti-isomers based on the respective synthetic procedures is proposed. [ABSTRACT FROM AUTHOR]

Abstract: Reaction of cis-[Mo2(OAc)2(CH3CN)6][BF4]2 with NP-Et,Me (2-ethyl-3-methyl-1,8-naphthyridine) in acetonitrile provides trans-[Mo2(NP-Et,Me)2(OAc)2(CH3CN)][BF4]2 (1). Partial protonation of 1 by HBF4·Et2O in acetonitrile leads to trans-[Mo2(NP-Et,Me)2(OAc)(CH3CN)3][BF4]3 (2). In both compounds, NP-R ligands are arranged in a head-to-head (HH) fashion leaving one of the axial sites vacant. Substitution of acetonitriles by NP-Me (3-methyl-1,8-naphthyridine) in trans-[Mo2(NP-tz)2(OAc)(CH3CN)2][BF4]3 provides trans-[Mo2(NP-tz)2(OAc)(NP-Me)][BF4]3 (3) with retention of configuration. Fully solvated dimolybdenum(II) compound reacts with NP-NH2 to provide [Mo2(NP-NH2)2(NP-NH)(CH3CN)2][BF4]3 (4) in which the NP-NH2 ligands are trans and arranged in a HH fashion. The deprotonated ligand (NP-NH−) binds the dimetal unit utilizing naphthyridine nitrogen and amido nitrogen. Treatment of [Mo2(NP-tz)2(CH3CN)4][CF3SO3]4 with bpym (2,2′-bipyrimidine) followed by crystallization in air provided an oxo complex [Mo2(NP-tz)2(μ 2-O)2(bpym)2][CF3SO3]4 (5). Compounds 1–5 have been characterized by a variety of spectroscopic techniques and by X-ray crystallography. The reactivity pattern is rationalized based on ligand labilities and thermodynamic stabilities. [ABSTRACT FROM AUTHOR]

Bimetallic alkylidene complexes of molybdenum (RF3O)2(ArN)Mo＝CH一SiMe2–CHMo(NAr)(ORF3)2(1) and (RF3O)2(ArN)Mo;SiPhVin–CHR=Mo(NAr)(ORF3)2(2) (Ar=2,6-Pr2iC6H3; RF3 =CMe2CF3) have been prepared by the reactions of vinyl silicon reagents Me2Si(CH＝CH2)2 and PhSi(CH ＝CH2)3 with known alkylidene compound PhMe2C–CH=Mo(NAr)(ORF3)2. Complexes 1 and 2 were structurally characterized. Ring opening metathesis polymerization (ROMP) of cyclooctene using compounds 1 and 2 as initiators led to the formation of high molecular weight polyoctenamers with predominant trans-units content in the case of 1 and predominant cis-units content in the case of 2. [Copyright &y& Elsevier]

Abstract: The syntheses and structures of homo- and heteronuclear biscarbene complexes with bithiophene spacers were investigated. The complexes were synthesized by lithiation of bithiophene followed by metallation using combinations of the metal precursors MnMeCp(CO)3, W(CO)6, Mo(CO)6 and Cr(CO)6, after which the reaction was quenched with triethyloxonium tetrafluoroborate. This classical Fischer method yielded monocarbene complexes, [ML n C(OEt)C4H2S–C4H3S], ([ML n ]=Cr(CO)5 1a, W(CO)5 2a or MnMeCp(CO)2 3a), homonuclear biscarbene complexes, [ML n C(OEt)C4H2S–C4H2SC(OEt)ML n ], ([ML n ]=Cr(CO)5 1b, W(CO)5 2b or MnMeCp(CO)2 3b) and heteronuclear biscarbene complexes, [ML n C(OEt)C4H2S–C4H2SC(OEt)M′L n ] (1d: [ML n ]=Cr(CO)5 and [M′L n ]=W(CO)5; 1e: [ML n ]=MnMeCp(CO)2 and [M′L n ]=Cr(CO)5; 1f: [ML n ]=Cr(CO)5 and [M′]=Mo(CO)5); 2d: [ML n ]=MnMeCp(CO)2 and [M′L n ]=W(CO)5; 3c: [ML n ]=MnMeCp(CO)2 and [M′L n ]=Mo(CO)5). Electron density calculations with the gaussian03 software package of 1e revealed a polar rod with the negative pole towards the chromium carbene side, whereas the biscarbenes 1d and 1b showed very little polarity. By-products resulting from activation of the carbene moieties in homonuclear biscarbene complexes included (i) ester-type complexes of the form [ML n C(OEt)C4H2S–C4H2SC(O)OEt], ([ML n ]=Cr(CO)5 1c or W(CO)5 2c), formed in situ in the reaction of 1b and 2b, (ii) the organic bis-ester compound [EtOC(O)C4H2S–C4H2SC(O)OEt] 4, where both metal moieties had been substituted by oxygen and (iii) the carbon–carbon coupled dimeric bithienyl compound [C4H3S–C4H2SC(O)C(O)C4H2S–C4H3S] 5. By-products obtained from heteronuclear biscarbene reactions contain the former diketo compound (or a derivative) as spacer between two metal carbonyl fragments and have the general formula [ML n C(OEt)C4H2S–C4H2SCR–CR′C4H2S–C4H2SC(OEt)ML n ] (5a: [M]=Cr(CO)5, R=OH, R′=OEt; 5b: [M]=W(CO)5, R=R′=O; 5c: [M]=Mo(CO)5, R=R′=O). Reaction of 1d, 1e and 1f with hex-3-yne resulted in the formation of benzannulated products 6a, 6b and 6c. All novel complexes were fully characterized using various spectroscopic techniques. The crystal structures of 1b, 2a and 5 are reported. [Copyright &y& Elsevier]

Abstract: A novel dioxomolybdenum (VI) complex of (ferrocenylmethyleneamino)alcoholate was easily synthesized via the reaction of MoO2Cl2(THF)2 with N-hydroxyethyl ferrocenylimine ligand [(η5-C5H5)Fe{(η5-C5H4)–CHCH2–CH2–OH}], and characterized by FR-IR, 1H NMR, UV–Vis spectroscopy and elemental analysis. In the presence of the catalytic amount of the dioxomolybdenum (VI) complex, good to excellent selectivities of epoxides (77–95%) were obtained for epoxidation of styrene and cyclohexene with tert-butyl hydroperoxide (TBHP) as oxidant in 1,2-dichloroethane at 12h. Higher initial activity of the complex was observed for the catalyzed styrene epoxidation (451molmolMo −1 h−1) than that of cyclohexene epoxidation (164molmolMo −1 h−1) at 1h. Meanwhile, influences of reaction temperature, solvents, reaction time as well as catalyst amount for olefin epoxidation were also investigated. [Copyright &y& Elsevier]

Abstract: A series of homologous mononuclear dioxomolybdenum complexes were prepared and fully characterized with structurally related thiosemicarbazone ligands supplying a tridentate O,N,S donor set to the central metal atom. The ligands are derived from the prototype 2-hydroxybenzaldehyde-4-triphenylmethylthiosemicarbazone (H2L). Within this series the crystal structures of 11 complex compounds [MoO2(LR n )(dmf)] and [MoO2(LR n )(MeOH)] were determined showing characteristic differences in the gross structural properties of the central metal core. From the variation of substituents in this ligand library the influences of electronic ligand effects on the spectroscopic, electrochemical, and functional properties of these biomimetic model complexes for molybdenum-containing oxotransferases are reported. [Copyright &y& Elsevier]

Bis(ferrocenyl)-substituted allenylidene complexes, [(CO)5M＝C＝C＝CFc2] (1a–c, F=(C5H4)Fe(C5H5), M=Cr (a), Mo (b), W (c)) were obtained by sequential reaction of Fc2＝CO with Me3Si–C≡CH, KF/MeOH, n-BuLi, and [(CO)5M(THF)]. For the synthesis of related mono(ferrocenyl)allenylidene chromium complexes, [(CO)5Cr＝C＝C＝C(Fc)R] (R=Ph, NMe2), three different routes were developed: (a) reaction of the deprotonated propargylic alcohol HC≡CC(Fc)(Ph)OH with [(CO)5Cr(THF)] followed by desoxygenation with Cl2C＝O, (b) Lewis acid induced alcohol elimination from alkenyl(alkoxy)carbene complexes, [(CO)5Cr＝C(OR)CH＝C(NMe2)Fc], and (c) replacement of OMe in [(CO)5Cr＝C＝C＝C(OMe)NMe2] by Fc. Complex 1a was also formed when the mono(ferrocenyl)allenylidene complex [(CO)5Cr＝C＝C＝C(Fc)NMe2] was treated first with Li[Fc] and the resulting adduct then with SiO2. The replacement route (c) was also applied to the synthesis of an allenylidene complex (7a) with a C≡C spacer in between the ferrocenyl unit and Cγ of the allenylidene ligand, [(CO)5Cr＝C＝C＝C(NMe2)-C≡CFc]. The related complex containing a CH＝CH spacer (9a) was prepared by condensation of [(CO)5Cr＝C＝C＝C(Me)NMe2] with formylferrocene in the presence of NEt3. The bis(ferrocenyl)-substituted allenylidene complexes 1a–c added HNMe2 across the Cα–Cβ bond to give alkenyl(dimethylamino)carbene complexes and reacted with diethylaminopropyne by regioselective insertion of the C≡C bond into the Cβ–Cγ bond to afford alkenyl(diethylamino)allenylidene complexes, [(CO)5M＝C＝C＝C(NEt2)CMe＝CFc2]. The structures of 5a, 7a, and 9a were established by X-ray diffraction studies. [Copyright &y& Elsevier]

Cyanide–isocyanide isomeric molybdenum(IV) complexes containing an end-capping Cr(CO)5 fragment were prepared from [IndCpMo(NCMe)2][BF4]2 or IndCpMo(CN)2. Substitution of the NCMe ligand in [IndCpMo(NCMe)2][BF4]2 (1) by cyanide anion CN− or [Cr(CO)5CN]− gave the complexes IndCpMo(CN)2 (4) and IndCpMo{(NC)Cr(CO)5}2 (3), respectively. The linkage isomer complex IndCpMo{(CN)Cr(CO)5}2 (6) was prepared by the reaction of IndCpMo(CN)2 with Cr(CO)5(THF). The crystal structure of 4 is built up from two independent molecules. In both, the molybdenum centre displays a pseudo-tetrahedral coordination sphere. Cyclic voltammetry revealed some electronic communication between the metal centres in the heterobimetallic complexes. DFT calculations (adf program) were performed in order to study the electronic structure of the complexes, the relative stabilities of the NC/CN isomers, and to understand the nature of the redox processes taking place. In all complexes, reduction leads to ring slippage, which may be more or less favourable and pronounced depending on the ligands surrounding the metal; in the trinuclear species 3 and 6 oxidation takes place on the Cr(CO)5 fragment. [Copyright &y& Elsevier]