Ligands non-bridging

Non-bridging ligands in electron transfer reactions. J. E. Earley, Prog. Inorg. Chem., 1970, 13, 243-262 (76). 

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

Anions such as PR T appear as bridging ligands, while PF4 has recently been identified as a non-bridging ligand in a few complexes. 

Figure 12 (a) Symmetrically and (b) asymmetrically bonded non-bridging ligands. 

The term pillared-layer implies that the 3D structure is hierarchical with the layer formed first and the pillar coimects later, but such phenomena can be hardly observed directly since the self-assembly and crystallization of MOFs are generally carried out in a one-pot reaction. Nevertheless, using suitable non-bridging ligands instead of the pillar ligands, the individual single layer structure can be sometimes synthesized. In an occasional case, the hypothetic monolayer and pillared double layer have been isolated during the synthesis of a pUlared-layer MOF [73]. 

More common coordination polymers consist of a mononuclear central atom with a bridging ligand. The CRU of such a potymer cites the central atom first, whidi is prefixed hy its associated non-bridging ligands, followed in turn by the name of the bridging ligand prefixed by the Greek letter p, ... 

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