Six-electron ligands

Ligands in this class are shown in Table 17. The complexes with benzene and substituted benzene ligands are considered separately. The generic name of the benzene ligands is arene . 

Ligand Metals with which the ligand forms complexes  

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D. Complexes Having Six-Electron Ligands I. Borazine (t)6-B3N3)... 

Pseudotetrahedral cyclopentadienyl derivatives that retain the metal-core bonding scheme commonly feature the C03CP3 moiety. Since each CpCo unit contributes one electron fewer than does (CO)3Co, stability requires that the apical group should be a six-electron ligand. Complexes that meet this requirement are of composition CP3C03 (arene), and are prepared thermally from CpCo(C2H4)2 (Section 5.1.2). The synthesis is only successful if the arene bears a vinyl group. ... 

Monomeric sulfur imide SNH is not known, but a coordination compound Fe2(CO)6(HNS) (72) has been isolated and converted to the corresponding anion [Fe2(CO)6(/r-SN)] with n-BuLi. The complex (72) is obtained from the reaction of Fe3(CO)i2 with MeySiNSNSiMes, followed by chromatography on silica. The HNS unit behaves as a six-electron ligand see Counting Electrons). Mononuclear complexes of the type (RNS)Cr(CO)5 (R = MeyN, PhyN), in which the ligand is bonded to the metal via sulfur, have also been structurally characterized. ... 

If a hexahapto benzene ligand does not coordinate to the metal atom by means of six 2c-2e metal-carbon bonds when it functions as a six-electron ligand, an alternative possibility is that it does so by donating its three pairs of K-bonding electrons, using them to form three three-center, two-electron (3c-2e) MC2 bonds. This might be written as shown in Equation (4.1) ... 

Transition-metal porphyrin complexes are found in such important proteins as hemoglobin, chlorophyll, and the cytochromes. Because of their biological significance, porphyrins and their simpler analogs have been extensively studied in the laboratory. Porphines and their close cousins the salens (salicylaldehyde eth-ylenediamine) are tetradentate, six-electron ligands, with two one-electron donors and two two-electron donors. Porphines are macrocycles made up of four pyrroles... 

In Nb3Cp3(CO)7 the bridging carbon monoxide is bonded to one niobium atom through carbon, and with the other two Nb atoms it forms a Tr-olefm bond (a six-electron ligand). 

The commonest six-electron ligands are benzene and substituted benzenes. Ligands of this type are termed arenes . They form complexes with most of the transition elements, although the variety of derivatives is somewhat smaller than for the cyclopentadienyls. Bis(benzene)complexes are known for M... 

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