Modes of coordination

As expected for a diphosphine ligand, dppf can coordinate to a metal center in several ways (Fig. 1-2). To adjust its mode of coordination or to relieve the strain imposed by complex formation, the Cp rings of dppf can twist about the Cp(centroid)-Fe-Cp(centroid) axis (Fig. 1-3) and tilt towards or away from the Fe center (Fig. 1-4). The phosphorus atoms provide added flexibility by diverging from coplanarity with the Cp rings (Fig. 1-5). With this motive freedom, the bite angle and transannular distance between the two donor atoms are adaptable to the 

Au2Cl2(/t-dppl) CHCls Molecule I open bridge antiperiplanar 180 

Ag2(A -C6H5C02)2(/t-dppl) quasi-closed bridge anticlinal (staggered) — 

Square planar cis-[Rh(dppf-P, P)(nbd)l[C10J chelate gauche (staggered)  

Rh2 (/2-S- Bu)2 (C0)2 (/2-dppf) quasi-closed bridge gauche eclipsed 72 

Trigonal planar [AuCl( t-dppl)] nCHjClz open bridge anticlinal (eclipsed) 153.1 

Simple ligands can adsorb on iron oxides to form a variety of surface species including mononuclear monodentate, mononuclear bidentate and binuclear mono or bi-dentate complexes (Fig. 11.2) these complexes may also be protonated. How adsorbed ligands (and cations) are coordinated to the oxide surface can be deduced from adsorption data, particularly from the area/adsorbed species and from coadsorption of protons. Spectroscopic techniques such as FTIR and EXAFS can provide further (often direct) information about the nature of the surfaces species and their mode of coordination. In another approach, the surface species which permit satisfactory modelling of the adsorption data are often assumed to predominate. As, however, the species chosen can depend upon the model being used, this method cannot provide an unequivocal indication of surface speciation confirmation by an experimental (preferably spectroscopic) technique is necessary. 

EXAFS data showed that cations and oxyanions (e.g. selenite and arsenite) can form two kinds of bidentate, inner sphere complexes on iron oxides depending upon the surface site at which the adsorbate adsorbs (Manceau, 1995 Randall et al.. 

For some of the most intensively studied anions, increased information has not clarified the speciation or coordination of the adsorbed complex. For example, on the basis of IR data it has been claimed that sulphate adsorbs on goethite as both a bidentate and a monodentate surface complex (Parfitt and Smart, 1977 Persson and Lovgren, 1996). Some of the conflict may arise from differences in the conditions under which the experiments were carried out. Most IR spectroscopy measurements of adsorbed sulphate on Fe oxides were made on dried samples. Hug (1997) compared 

Adsorption of phosphate is initially rapid and is followed by a slow stage (hours to days) that is more pronounced for less crystalline samples of Fe oxides (Barrow et ak, 1981 Torrent et ak, 1990 Nilsson et ak, 1992). The slow stage has been attributed to diffusion into micropores or grooves (Torrent, 1991 Strauss et ak, 1997) and into aggregates of particles (Anderson et ak, 1985 Willet et ak, 1988). Evidence for slow dif- 

Adsorption of phosphate on Fe oxides involves a ligand exchange mechanism (Par-fitt and Russell, 1977 Sigg and Stumm, 1981) and appears to be promoted by increasing the ionic strength (Bowden et al., 1980). Spectroscopic studies have not provided an entirely consistent picture of the mode of phosphate adsorption, but the consensus from studies with a range of techniques is, that phosphate adsorbs on Fe oxides predominantly as a binuclear, bidentate complex. 

---

The nitrate group, like the nitrite group, is a versatile ligand and numerous modes of coordination have been found in nitrato complexes. ° The uncoordinated N03 ion (isoelectronic with BF3, B03 , C03 , etc.) is planar with N-O near 122 pm this value... 

At least 8 modes of coordination are known (Table 15.9) - they are all based on either side-on S2 or bridging -S-S- with possible further ligation via one or two lone-pairs as shown schematically below ... 

Figure 15.30 Structure of the thiosulfate ion and its various modes of coordination (a) uncoordinated...

T) (N, N ) mode complexes. Suspected T) -mode of coordination still remains unconfirmed. The organometallic chemistry of pentazole is the challenge for the further developments. 

The first region of maximum electron density is along the lone-pair directions, and the second is in the Qy plane, parallel to the axis. Thus, two possible modes of coordination arise, these are shown in Fig. 11, which illustrates the proposed bonding in Hb02-... 

A novel mode of coordination of diphosphenes is seen in the ligand... 

The importance of alkali metal binding with available 7r-electron density in the formation of CIPs was also demonstrated by Niemeyer in the structural elucidation of the first monomeric non-solvated lithium cuprate, [(2,6-Mcs2(LI L)2CuLi] 450, formed from the reaction of 2 equiv. of (2,6-Mcs2Gf,I L)Li with /-BuOCu in pentane.447 The complex crystallizes as two different independent molecules in which the C-Cu-C angles differ (171.1° and 173.8°) as does the mode of coordination to the Li cations C pso and rf to one pendant Ph in molecule 1, with an additional rf interaction to a second Ph group in molecule 2. In the second molecule, the Li site is 10% occupied by a Cu ion. 

Consequences of unsaturation. Unsaturation in the macrocyclic ring may have major steric and electronic consequences for the nature of the ring. Extensive unsaturation will result in loss of flexibility with a corresponding restriction of the number of possible modes of coordination. Further, loss of flexibility tends to be reflected in an enhanced macrocyclic effect . For example, if the metal ion is contained in the macrocyclic cavity, the loss of flexibility reduces the possible pathways for ligand dissociation and this tends to increase the kinetic stability of the system. As explained in later chapters, enhanced thermodynamic stabilities will usually also result. 

Monomeric sulfur diimides have an extensive coordination chemistry, as might be anticipated from the availability of three potential donor sites and two re-bonds.131 Under mild conditions with suitable coordinatively unsaturated metal complexes, sulfur diimides may coordinate without rupture of the -N=S=N- unit. Four modes of coordination have been identified or invoked as intermediates in fluxional processes (Scheme 8). 

In homogeneous solutions NO disproportionation maybe promoted by transition metal complexes, and a variety of mechanisms seem to be available owing to the many possible modes of coordination. One example is the reaction of NO with nickel carbonyl shown in Eq. (35) (76),... 

Chemisorption Adsorbate Orientation Competitive Chemisorption Adsorbate Exchange Adsorbate Reactivity Electrocatalvsis Synthesis Mode of Coordination Ligand Substitution Ligand Exchange Ligand Reactivity Homoqeneous Catalysis... 

An oxygen-bonded sulfoxide complex will be termed anO-RjSO complex, and similarly, a sulfur-bonded complex will be termed an S-RjSO complex. Where the mode of coordination is not known or is uncertain, formulas of the type [M(R2SO)xC1 ] will be used. 

---