Unique bridges

Over the course of the last several years, essential studies have been conducted in a wide variety of areas using supercritical fluids. The continued constructive development of the science employing these fluids is dependent upon a unique bridging of scientists from many disciplines involved in this work. This text, a compilation of several of the papers presented at the ACS meeting in Atlanta in the spring of 1991, attempts to illustrate the numerous scientific endeavors that have been and continue to be pursued. Individually, these works are able to stand alone in their viability. Collectively, they demonstrate the vast interest, the overwhelming potential and the extensive growth that is available. 

Some bond distances are given in Table 5. The B—Hterminal distances not listed are normal. The structure, in solution, is assumed to be stereochemically non-rigid a fluxional process occurs with the unique bridging hydrogen moving between B2 and B5. This assumption is based on the nB NMR spectrum which indicates Cs symmetry116 . 

The presence of simple ir-allyl and ir-cyclopentadienyl ligands symmetrically coordinated to an M3 unit seems to be unique. Bridging allyl groups are found in [C3H5Pt(acac)]2 (42) and in the tetrameric [C3H5PtCl]4 (43), but they are linked differently to the platinum atoms, namely mono-hapto to one platinum and dihapto to the other. There is also no metal-metal bond formed in these complexes. 

The benzidine rearrangement was employed recently for the preparation of unique bridged diaminodiphenyls 130 and 131227,228. An unprecedented [9,9]-sigmatropic shift was discovered in the acid-catalyzed benzidine rearrangement of bis[4-(2-furyl)phenyl]... 

A further reaction of the starting cluster is the coordination of /7A(dimethyl)ace-tylene dicarboxylate, which again involves the unique bridging hydride. In this case, both of the cluster hydrides are transferred to the same acetylenic carbon of the alkyne, so that the coordination of the resulting dihydro-derivative is through a single carbon, giving the carbene-complex with the structure shown in Fig. 21. 

Reaction of CO with the tautomeric mixture of the two aforementioned rhodium complexes (several / flra-substituted imidoaryl groups were tested) afforded a unique bridging isocyanate complex Rh2(CO)2(ii -N,T] -C, x-ArNCO)(p-DPPM)2. The CO insertion is irreversible. Since the two initial tautomers are in equilibrium in solution, insertion of CO may in principle proceed by either of the two (Scheme 20)(next page). However, evidence was given in favour of the amido-path (path b in the Scheme), based on the fact that the cationic complex [Rh2(p-NHPh)(CO)2(DPPM)2] rapidly reacted with CO. No complex could be isolated from this last reaction, but the formation of PhNCO was detected. Two features of this mechanism are worth of note. The first is the contrast between the conclusion reached for this system (amido complex more reactive than imido one in the insertion reaction of CO) and the one reached by Bhaduri et al. [161] for the trinuclear complex Ru3(p-H)(p-NHPh)(CO)io, which, upon deprotonation of the amido group by OH, affords the inserted product [Ru3(p-H)(T] -N,ii -C,p3-PhNCO)(CO)9]. The difference is likely due to the fact that, in this latter case, the complex is trinuclear, so that the inserted CO is already coordinated to the third ruthenium atom and, especially, the formation of the new C-N bond does not require the breaking of any of the pre-existing Ru-N bonds. 

Composites have been used as superstructural elements as well as substnictural elements of bridge structures. Composites are ideally suited for use in unique bridges, such as floating or movable bridges. 

When a solution of diphenylketene and Fe(CO)j in benzene are irradiated the complex, 1.19a, is formed. The crystal structure shows that a unique bridging carbon system is formed [98A],... 

---