Lantern dimers

Synthesis and molecular structure of a Lantern dimer [AsPhJJTc OJSCH CONHCHfiH NHCOCH S)J. 123 ... 

Both of these emissive excited states are stabilized by intermetallic bonding. For these A-frame type complexes, however, there is a second pair of singlet and triplet excited states derived from the monomer -> pz excitation. This second pair of states are derived from transitions that are allowed in the C2v symmetry of the A-frame dimers, but not in the D h symmetry of the lantern dimers. These latter states are not stabilized by intermetallic bonding, but have instead considerable MLCT character. 

Figure 2.35 The lantern structure adopted by dimeric rhodium(II) carboxylates.

The lantern structure is quite stable, unlike certain other Rh2 dimers. Protonation was formerly claimed to give Rh2+ aqua ions, but they are believed now to be [Rh2(OCOMe)3]+ (aq.) and [Rh2(OCOMe)2]2+ (aq.) [73],... 

Figure 2.39 The lantern structure of the dimeric rhodium antipyrine complex Rh2(ap)4Cl. (Reprinted with permission from Inorg. Chem., 1988, 27, 3783. Copyright (1988) American...

Platinum(III) complexes are remarkably rare and most examples date from less than 30 years ago. The great majority are dimeric species with only a few authenticated stable monomers reported. Most of the dimers are bridged by bidentate ligands and are almost exclusively doubly or quadruply bridged, the latter giving rise to the classic lantern structure. 

Unsolvated [Rh2(OCOR)4]2 can be obtained by sublimation. The lantern structure is retained with the axial position occupied by oxygens from neighbouring dimer units. The presence of axial ligands has little effect on the Rh-Rh bond therefore, in [Rh2(OCOCF3)4] Rh-Rh is 2.382 A compared with 2.394 A in [Rh2(OCOCF3)4(H202)] and 2.418 A in Rh2(OCOCF3)4 (MeCN)2 [74],... 

Before turning to specific results we will have a look at the properties of rhodium(II) acetates/carboxamidates as catalysts for reactions with diazocompounds as the substrates via carbenoid intermediates. Rhodium(II) has a d7 electron configuration, forming the lantern type dimers with bridging carboxylates. The single electrons in the respective dz2 orbitals form an electron... 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

Another widespread type of metal binding of carboxylate ligands is that in which the formation of dimers occurs having the structure of a Chinese lantern 283 [4, p.l32 537] ... 

Despite the vast number of cobalt(IT) complexes known, the dipositive oxidation state is rare for rhodium. The bulk of the rhodium(ri) complexes known are dimeric, the classic examples being the diamagnetic carboxylato complexes that adopt the classic lantern structure (26). However, even the aqua complex is dimeric so the bridging carboxylato ligands are not essential for the formation of the rhodium-rhodium bond. 

Refluxing RuC13.a H20 with ethanoic acid/ethanoic anhydride mixtures, particularly in the presence of LiCl, gives a green solution and crystals of the dimeric Ru2(OAc)4Cl, an unusual mixed-valence compound with a lantern structure (Figure 1.32) where the dimer units are linked into continuous chains by axial bridging chlorides (the bridges are sometimes kinked, sometimes linear) [96]. 

Triply bonded dimers where Nb has a figuration could also be stabilized with anioiric ligands having N-C-N moieties such as 7-azaindolyl or hpp (l,3,4,6,7,8-hexahydropyrimido[l,2-a]pyrimidinate). The tetragonal lantern Nb2(hpp)4 compound (50) with a Nb-Nb distance of 2.035(9) A represents the shortest triple bond reported so far for niobium. This distance is much shorter than that found in the metal (2.85 A). Attempts to reduce (50)... 

The electronic structures of lantern-type Pt(III) dimers are discussed on the basis of a general scheme for dimers with a d -d electronic configuration, as depicted in Fig. 20. HOMO is considered either the diT or the c 8 orbital of the Pt—Pt single bond. The LUMO is usually do- orbital. In the case of the Pt(II) dimers, HOMO is now dcr orbital... 

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