Bridging modes

By far the most common CN of hydrogen is 1, as in HCl, H2S, PH3, CH4 and most other covalent hydrides and organic compounds. Bridging modes in which the H atom has a higher CN are shown schematically in the next column — in these structures M is typically a transition metal but, particularly in the Mi-tnode and to some extent in the x3-mode, one or more of the M can represent a main-group element such as B, Al C, Si N etc. Typical examples are in Table 3.3. Fuller discussion and references, when appropriate, will be found in later chapters dealing with the individual elements concerned. 

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

The oxygen-bridging mode (v) is less common but occurs together with modes (iii) and (iv) in the following centrosymmetrieal trimeric Ni complex and related compounds. 

It is possible that a sixth (symmetrical bridging) mode M—O O—M occurs in... 

Figure 14.4 The four main types of O2-M geometry. The bridging modes Ib and lib appear superficially similar but differ markedly in dihedral angles and other bonding properties. See also footnote to Table 14.5 for the recently established unique /Lt.rj -superoxide bridging mode.

MCl3(NS)(PPh3)2]. In the few complexes for which X-ray structural data are available the M-N-S group is essentially linear (170-177°) (see p. 453 and refs. 233, 235) but spectroscopic data on others suggest that bent and even ij -bridging modes may be possible. 

Ti in [Ti(> 2-Cl04)4] and Ni" in [Ni(>j -C104)L2] where L is a chiral bidenlate organic ligand.Sometimes both and modes occur in the same compound. The biden-latc bridging mode occurs in the silver complex [Ag /x,>j -00(0)20- (m-xylene)2]- The structure of appropriate segments of some of these compounds arc in Fig. 17.23. The distinction between coordinated and non-coordinaied ( ionic ) perchlorate is sometimes hard to make and there is an almost continuous... 

Inter [4+2]/intra [3+2] This typeof tandem reaction nsmg nitroalkenes hasbeen expbred most extensively Four subfamilies of tandem cycloaddiQon exist, which anse from the font different pomts of attachment of the dipolarophihc tether They are defined as fused, spiro, and bridged modes, as depicted m Scheme 8 37... 

A unique pair of stereoisomeric dimolybdenum amidinate complexes has been prepared and structurally characterized. The reaction of Li[PhC(NSiMe3)2] with dimolybdenum tetraacetate afforded trans- and ds-Mo2(02CMe)2[PhC (NSiMe3)2]2- While the acetates coordinate to the M02 core via a bridging mode in both compounds, the benzamidinates are bridging in the trans complex and... 

For the purines, only in the case of adenine has the [N3 + N9] bridging mode been observed (76-78). This further highlights the difference in binding patterns between the N3 of A and G. A recent example is seen in the Pd2-adenine complex, 18, formed in reactions of 19 with... 

Second only to sulfur-based systems, nitrogen complexes are relatively well represented in the structural literature with 41 complexes reported. Of these, 25 are with I2 as the electron acceptor, 11 are with the interhalogen IC1, three are with Br2, and two are with IBr. As expected, in every case the halogen bond forms between the nitrogen and the softest halogen atom, i.e., iodine, in all of the complexes except those with dibromine. Most N I2 complexes, and all N Br2, N IBr, and N IC1 complexes are simple adducts, mode A. Exceptions for the diiodine complexes include bridging mode (B) observed for diazines, such as pyrazine [86], tetramethylpyrazine [86], phenazine, and quinoxaline [87], and for 9-chloroacridine [89] and the 1 1 complex of diiodine with hexamethylenetetramine [144] and amphoteric bridging mode (BA) observed for 2,2 -bipyridine [85], acridine [89], 9-chloroacridine [89], and 2,3,5,6-tetra-2/-pyridylpyrazine [91]. The occurrence of both B and BA complexes with 9-chloroacridine, and of B and A complexes and an... 

Carbonate is a well-established ligand, but its coordination diversity continues to surprise. Elaborate bridging modes have been reported for carbonate for example, a /i4-carbonato-bridged Co11 compound [Co2(2,2 -bipyriniidinc)(0[ I2)2(/i-C03)(//-OI I)]1 has been characterized.429... 

Acetate remains the classical carboxylate ligand, with the capacity to bind in monodentate bidentate chelate (/i2), or bridging modes. Whereas early chemistry featured simple fii... 

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