Unsymmetrical bridging

BgH22, and nonaborane(15) [19465-30-6] can generally be cleaved by nucleophiles in two ways termed symmetrical and unsymmetrical bridge... 

Examples of the unsymmetrical bridging mode (iv) are shown in the top diagram. 

The zinc complex of a 14-membered CA-N2S2 dibenzo macrocycle with two pendent pyridyl-methyl groups was studied (L = 8,ll-bis(2-pyridylmethyl)5,6,7,8,9,10,16,17-octahydro-diben-zo[e,m][l,4]dithia[8,ll]diazacyclotetradecine). In solution, [ZnL]2+ takes up atmospheric carbon dioxide and transforms to the dinuclear complex [(//-C03)(ZnL)2](C104)2. The structures of both complexes have been elucidated. The zinc atoms, which are situated outside the macrocyclic ring, are unsymmetrically bridged by the //-carbonato group.739... 

The tin(II) derivative Sn[S2P(OPh)2]2 is a dimeric supermolecule formed through intermolecular Sn- S secondary bonds and Sn-7i-aryl interactions.57 Lead(II) dithiophosphates, Pb[S2P(OR)2]2, e.g. R = Me,58 Pr" and Cy,59 all form supramolecular chain-like arrays. The ligands are unsymmetrically bridging and form Pb- S secondary bonds. 

Ph3PbS2PMe2]x (with unsymmetrical bridging dithio ligands and secondary Pb S bonds),168 and monomeric Ph2Pb(S2PPh2)2 (six-coordinate lead, aniso-bidentate ligands).169... 

A relatively unexplored method to prepare a pyridine ring by thermolysis of O-allyl cyclic oximes (593) was used in the synthesis of some unsymmetrical bridged terpyridines and in the synthesis of the E-ring in modified steroids" (equation 261). 

A second class of unsymmetrically bridged systems is represented by pentamethyl(diphenylamino)dialuminum in which the bridging atoms are different, one providing an electron-deficient bridge bond and the other a normal bridge with two electron bonds (11, 40, 43, 51, 57, 59, 61, 71, 72, 110). The only species of the latter type for which structure has been... 

Bridging is indicated by the i prefix before the bridging ligand name, and unsymmetrical bridging by multiatom bridges is denoted by the ligating atom notation and central atom locant number separated by a colon x-(hydroxyamido)-2idV lK0-. For examples see Table 18. 

Face-bridging ligands hexahedro-fg3-Xjg-octahedro-A/p 4.6.83 Unsymmetrically bridged octahedro-A/g... 

The methylene isomer 15 eliminates carbon monoxide when heated at 110°C (toluene, xylene 24 h), and yet another carbon-metal hydrogen shift takes place (75). The methylidyne species 16formed in this irreversible reaction has been characterized by its NMR spectra [S(CH) 9.36, quartet (76) 8(CH) 118.4 (CD3)2CO (77)] which implies a structure analogous to the previously known ethylidyne derivative (/i.-H)3(/i.-CCH3)-Os3(CO)io (138). NMR data obtained for partially deuterated samples of the methyl complex 14 have yielded evidence that the methyl ligand occupies an unsymmetrical bridging position between two metal centers, a coordination mode which implies significant C H Os interaction... 

The reaction of H2Os3(CO)10 with Rh(C2H4)2(MeCOCHCOMe) yields the mixed metal cluster RhOsjH2(MeCOCHCOMe)(CO)10 whose structure is shown in Figure 15. One of the acetyl-acetonate oxygen atoms, 0(3), unsymmetrically bridges Os(3) and Rh, while the other, 0(2), is coordinated only to rhodium so that this ligand can be considered as a five-electron donor.509... 

Disulfido ligands of structure type lb also exist in the interesting compound (NH4 )4 [Mo4 (N0)4 (S2 )5 S3 ] 2H2 O (11), which contains sulfur atoms in five different bonding situations. Four of the five S2 ligands unsymmetrically bridge pairs of Mo atoms (structure type lb). One S ligand is side-on coordinated to a single Mo atom (structure type la). 

A number of M(/i-H) M systems are known. These necessarily have both a bent M—H—M arrangement and, in view of the small covalent radius of H, a short M—M distance. Such systems also occur in metal clusters, e.g. the intensively studied Os3H2(CO)10. A strongly unsymmetrical bridge has been observed in [(dpe)Pt(/i-H)2PtH(dpe)]+ by neutron diffraction.8... 

At the extremes, the spectra of the di-cr and n complexes are clearly distinguishable as types I and II, respectively. The type I spectra are intermediate in form thus we have suggested that these may form a continuous series with type II spectra for C2H4 bonded to a single metal atom. If this is the case, in due course other examples may be found to bridge the remaining gap between type T and type I spectra, i.e., with unsymmetrically bridged species. However, there remains the possibility that in some cases the observation of type I rather than type I spectra is due to experimentally variable impact contributions (Section IV.B). 

The unsymmetrically bridged ions 506 would be undistinguishable from the symmetrically bridged system 504 in solution NMR experiments. (However, see subsequent discussion of solid-state low-temperature 13C NMR as well as ESCA studies.) It is important to recognize that equilibrating open classical cations 505 cannot explain the NMR data and thus cannot be involved as populated species. 

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