Diatomic ligands

Fig. 2. Stereo view of the active site off), gigas hydrogenase (reprinted with permission from (65) copyright 1997, American Chemical Society). LI and L2 are diatomic ligands that form hydrogen bonds with the protein- they are supposed to be the two CN s molecules. The third ligand L3 sits in a hydrophobic pocket and is assumed to be the CO. The designates the putative oxo bridging ligand.

FIGURE 16.8 Molecular orbital diagram for some common diatomic ligands. 

Figure 15.3 The dinuclear Ni-Fe site of the hydrogenase from D. norvegium in the reduced form. The diatomic ligands bound to the Fe are CO and two CN molecules Ni, green Fe, pint, S, yellow, and Se, light blue. (From Mulrooney and Hausinger 2003. Copyright 2003, with permission from Elsevier.)...

Figure 6.10 The catalytic site of [NiFe] and [NiFeSe] hydrogenases in oxidised inactive (top) and reduced active (bottom) states. Note the three non-protein diatomic ligands to the iron.The site bridging the Ni and Fe is occupied by an oxygen or sulfur species in the most oxidised states and probably by a hydride or molecular hydrogen in the most reduced states.

Each iron atom is terminally coordinated by one CO and one CN . The coordination of Fel is completed by the additional bridging cysteinyl sulfur. For Fe2, the sixth coordination site may be empty, as in the D. desulfuricans enzyme (Nicolet et al. 1999), or occupied by a solvent molecule, as observed in the C. pasteurianum enzyme (Peters et al. 1999). The assignment for the diatomic ligands is supported by infrared spectroscopic evidence (Pierik et al. 1998X and similar diatomic ligands have also been found for the corresponding binuclear [Ni-Fe] cluster of the nickel-iron hydrogenases (Volbeda et al. 1995). 

Conspicuously absent in HO-1 is any residue analogous to the globin and peroxidase distal His that can directly interact with iron-linked ligands. Nevertheless, the HO-1 distal pocket is polar (Fig. 20) with the potential for diatomic ligands to interact with the kinked region of the distal helix, as well as water molecules that could bridge between... 

Fig. 20. A hypothetical model of one possible binding mode for a diatomic ligand to HO-1. Both Aspl40 and Argl36 should interact with a ligand via an ordered water molecule. Whether or not these residues are critical in an arid-base catalytic process or are simply used to sterically orient an iron-linked peroxo intermediate remains unknown.

Many complexes with stable homonuclear diatomic ligands like Oj and N2 are well known and have been extensively studied. Only in recent years has the chemistry of metal complexes containing one or more coordinated homonuclear sulfur ligands, (with ra = 2 or n > 2), been developed systematically. Complexes with units can be obtained for many metals under a variety of conditions, and coordinated ligands exhibit an especially rich structural chemistry. 

Predictions of the Pi parameter for some (hypothetical) unsaturated diatomic ligands (e.g. SO +, NS+, PO+, CS, or CP ) were also proposed ]49] on the basis of linear correlations between experimentally determined Pi values and quantum-chemical MO indices for uncoordinated... 

Many diatomic ligands favor end-on coordination to a single metal atom. This mode of coordination is also known for several 02 complexes, although doubly bridging end-on coordination (structural types Ila and lib) is more common. For the ligand only type Ha and type lib structures are known. 

One framework for discussing the bonding of metal complexes with diatomic ligands is to partition the complex conceptually into the units Mm+ and YJ". The values for m and n are determined by comparing the physical properties of the complex (Y—Y distance, v(Y—Y) (band position and intensity), electronic spectra, XP spectra, magnetic properties) with those of the isolated Y2" species. 

Transition metal porphyrins with diatomic ligands have received particular attention in view of their special relevance to biological systems. These diatomic ligands include 02 (obviously, see Section 62.1.12), CO,603,604 CS605 and no.606,607 The IR and resonance Raman spectra of porphyrins with these axial ligands have been well studied, for example Co11 tetraphenylporphyrin.608... 

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