Mononuclear compounds

Divalent molybdenum compounds occur in mononuclear, dinuclear, and hexanuclear forms. Selected examples are shown in Figure 6. The mononuclear compounds are mostiy in the realm of organometaUic chemistry (30—32). Seven-coordinate complexes are common and include MoX2(CO)2(PR3)2, where X = Cl, Br, and I, and R = alkyl MoCl2(P(CH3)3)4, heptakis(isonitrile) complexes of the form Mo(CNR) 2 (Fig. 6d), and their chloro-substituted derivatives, eg, Mo(CNR)3CR. The latter undergo reductive coupling to form C—C bonds in the molybdenum coordination sphere (33). 

A mononuclear compound containing gold in a formal oxidation state of +4 is shown in Figure 4.29 it was produced by electrochemical oxidation of the related gold(III) species [142],... 

Chelation of ixo-maleonitriledithiolate (imdt) has been structurally characterized in the octahedral cobalt(III) complex trmw-[Co(imdt)2(P(ra-Bu)3)2], formed via reaction of cobalt(II) ion with K2(imdt) in the presence of the phosphine.1037 Simple chelating thiolates such as SCH2CH2S not only form mononuclear compounds, but participate in bridging in clusters such as [Co3(SCH2CH2S)3(PEt3)3]3+ (243).1038... 

The compound [Pd[/i2-(C6H4)PPh2]Brj 4 has [(C6II4)PPh2] anions bridging two palladium atoms in a tetranuclear structure. This tetranuclear frame was cleaved with phosphines to form mononuclear compounds.568 Intermediate products of the frame degradation were detected (see Figure 50).567... 

Trithiocyanuric acid gives the trinuclear derivatives [Au3(S3C3N3)L3] (L = PPh3, Bu NC, P(OMe)3, P(OPh)3, PCy3), that by loss of one ligand L yields the hexanuclear species [Au6(S3C3N3)L4] (536) (L = PPhMe2, Bu NC).2960,2961 The mononuclear compound... 

Mononuclear compounds of the type [Au(S2CNR)fPPh3)2] (R = pip, pr )2979>2980 and [Au(S2C-NEt2)(PR3)] (PR3 = PCy3, P(p-MeOC6H4)3)2, 2982 have been structurally characterized. Mono-... 

Oxazolinyl-thien-5-yl- and 2-oxazolinyl-thien-3-yl-lithium react with (tht)AuCl to produce the corresponding gold complexes as a dimer and a trimer, respectively. Mononuclear compounds are obtained with (tht)AuC6F5 and (Ph3P)AuCl (Scheme 68).269... 

Ligand Preferences. The LVC s are electron rich and in order to exist in stable compounds they require ligands, such as CO, that are weak donors and good tt acceptors. On the other hand the HVC clusters are not attractive to such ligands nor do they require them for stability. In this respect, there is a direct parallel to mononuclear compounds where M° prefers CO, RNC or similarly ir-acidic ligands while the Mn+ (n = 2-4) ions do not generally form CO complexes. 

The E-ligand bond lengths, including those for bulky ligands, match those found in simple mononuclear compounds (Tables 2 and 3). The E—E bond lengths may be compared with values for tetrahedral E in the elements (pm) ... 

Abstract In this chapter, selected results obtained so far on Fe(II) spin crossover compounds of 1,2,4-triazole, isoxazole and tetrazole derivatives are summarized and analysed. These materials include the only compounds known to have Fe(II)N6 spin crossover chromophores consisting of six chemically identical heterocyclic ligands. Particular attention is paid to the coordination modes for substituted 1,2,4-triazole derivatives towards Fe(II) resulting in polynuclear and mononuclear compounds exhibiting Fe(II) spin transitions. Furthermore, the physical properties of mononuclear Fe(II) isoxazole and 1-alkyl-tetrazole compounds are discussed in relation to their structures. It will also be shown that the use of a,p- and a,C0-bis(tetrazol-l-yl)alkane type ligands allowed a novel strategy towards obtaining polynuclear Fe(II) spin crossover materials. 

The N-l coordination mode has been found in bidimensional- [3] and in tridimensional materials [4] derived from 4,4 -bis-1,2,4-triazole, as well as in mononuclear compounds of bidentate 1,2,4-triazole ligands in which the N-4 atom is protected from coordination by a non-coordinating substituent, as in 4-amino-3,5-bis(pyridin-2-yl)-l,2,4-triazole [5-8], 4-p-methylphenyl-3,5-bis(pyridin-2-yl)-1,2,4-triazole [9] and 4-ra-methylphenyl-3,5-bis(pyridin-2-yl)-l,2,4-triazole [9]. 

The mononuclear compound 8 and a related dimer 6 were characterized in the early stages of this research (2,3,37). The two forms can be interconverted by the net reaction... 

An interesting effect of pH was found by Ogo et al. when studying the hydrogenation of olefins and carbonyl compounds with [Cp Ir(H20)3] (Cp = ri -CsMej) [89]. This complex is active only in strongly acidic solutions. From the pH-dependence ofthe HNMR spectra it was concluded that at pH 2.8 the initial mononuclear compound was reversibly converted to the known dinuclear complex [(Cp Ir)2(p-OH)3] which is inactive for hydrogenation. In the strongly acidic solutions (e.g. 1 M HCIO4) protonation of the substrate olefins and carbonyl compounds is also likely to influence the rate ofthe reactions. 

B. Mononuclear Compounds of Iron, Molybdenum, Tungsten, Rhenium, Platinum,... 

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