Coordination sites, blocking

If suitable molecular analogues for these components can be found, then the spontaneous formation of a square complex can be designed. A 90° angle is frequently created from a square-planar metal ion, such as Pd or Pt, which has two cis coordination sites blocked off. The linear sides require a ligand that has metal binding sites directly opposite each other, such as 4,4 -bipyridine. 

First, dehydrogenative bonding of acetylene to the catalyst surface will free hydrogen and produce moieties bonded to the catalyst coordination sites. These units are assumed to be the building blocks for the tubules. 

Other isomers of cdt are also obtained and, if a coordination site on the nickel is blocked by the addition of a ligand such as a tertiary phosphine, dimerization of the butadiene, rather than trimerization, occurs. 

The rhodium(III) triaryls have pseudo-octahedral structures therefore, in the air-stable trimesityl rhodium, the three mesityl groups are arranged in /ac-positions, with ort/io-methyls blocking the other coordination sites (Figure 2.107). 

This compound will not polymerize ethylene or styrene, suggesting the coordination sites are blocked by the pyridine. The most probable structure for this compound is an octahedral one, the trans form of which is represented by... 

The trans/cis ratio of hexadiene is also increased by addition of aluminum cocatalysts containing —OR or —NR groups. It is proposed that this is due to selective blocking of a coordination site on the nickel by the formation of an intermediate such as 31, and this too would favor the formation of the transoid structure. 

The predominant binding site in 9-substituted 6-oxopurines (guanine and hypoxanthine derivatives) is the N7 atom of the base (Figure 6). The prevailing keto tautomer requires proton at N1 even in mildly acidic conditions, which efficiently prevents platination of the N1 site [7]. Under neutral and basic conditions competition of Pt(II) between the N1 and N7 sites has been reported. Attachment of Pt(II) to the N7 atom acidifies the N1H proton and facilitates coordination of additional platinum ions to both N1 and N3 [7]. In N7,N9-blocked 6-oxopurines, the N1 site is the major coordination site [7,24],... 

Monomethylation of tpy blocks one A-coordination site, and, depending on the reaction conditions, VV-Metpy+ coordinates to Ru either as an N,N - or A,A, C-donor. ... 

In some cases, it is possible to promote the Diels-Alder-type reaction. We have found that in the presence of polar auxiliary ligands (stabilizer), the Co-mediated reaction of alkynes with activated C,C or C,0 double bonds takes place (87MI6). These findings are only understandable if there is a direct interaction between the cp-cobaltacyclopentadiene and the polar stabilizer, which may act by blocking free coordination sites at the Co [Eq.(45)]. 

Fig. 1.39 Simplified structure of RuCljCPPhj) based on [883], the phenyl ring partly blocking the vacant sixth coordination site is shown...

It is made by reaction of RuClj with PPh in methanol, and forms shiny black crystals [881, 882], The X-ray crystal structure shows the molecule to have a distorted square-based pyramidal structure, a phosphorus atom forming the axial bond (Ru-P 2.230(8) A) while the basal plane has two trans chloro ligands (Ru-C1 2.387(7) and 2.388(7) A) and two trans phosphine ligands (Ru-P 2.374(4) and 2.412(6) A). It can be regarded as octahedral, with the sixth position blocked by a phenyl ring (Fig. 1.39) [883]. The catalytic efficacy of the complex may well depend on the availability of this vacant coordination site. 

---