Formals, unsaturated

Carbon atoms can also form cyclic compounds. Aromatic hydrocarbons (arenes), of which benzene is the parent, consist of a cyclic arrangement of formally unsaturated carbons, which, however, give a stabilized (in contrast to their hypothetical cyclopolyenes), delocalized system. 

Bonding. All phosphazenes, whether cyclic or chain, contain the formally unsaturated... 

Symmetrical triazines, although not formally unsaturated, in the presence of the dialkyl phosphites undergo ring opening and selfcatalysis for Pudovik-type addition.318-322 An example is shown in Equation 3.19. 

The original concepts of metal-ligand bonding were essentially related to the dative covalent bond the development of organometallic chemistry has revealed a further way in which ligands can supply more than one electron pair to a central atom. This is exemplified by the classical cases of bis(benzene)chromium and bis(cyclopentadienyl)iron, trivial name ferrocene. These molecules are characterised by the bonding of a formally unsaturated system (in the organic chemistry sense, but expanded to include aromatic systems) to a central atom, usually a metal atom. 

The addition of transition metal fragments ML (L = two-electron donor ligand) across formally unsaturated metal-metal or metal-carbon bonds is a well-developed synthetic route to heteronuclear clusters (1,2,11,12,27) and has received theoretical justification from Hoffmann s isolobal principle (46). The addition of a PtL2 fragment across an M=M double bond may be considered as analogous to the reaction of a carbene with an olefin, resulting in a cyclopropane. The use of isolobal analogies in the directed synthesis of heteronuclear clusters has been reviewed (11,12,27). 

All phosphazenes, whether cyclic or chain-like, contain the formally unsaturated group P=N with four-coordinate P and two-coordinate N atoms. Based on available experimental data, the following generalizations may be made in regard to their structure and properties ... 

Other examples of formally unsaturated clusters are provided by the series Os4(CO)i2(AuYR3) (Y = P, As R3 = Et3, Ph2Me, Ph3> (328). Some of their reactions are depicted in Scheme 30. A parallel can be drawn between these unsaturated clusters and Os3(CO)i(,H2, which has been used for the synthesis of a multitude of OS3 derivatives. 

Clusters, particularly those described in Section 2.2 as orbitally rich, can act as Lewis acids. Addition of a Lewis base formally adds two electrons to the cluster bonding system. For some clusters, this causes an opening of the cluster, for example, closo to nido interconversion of B3HgMn(CO)3 and B3Fl8Mn(CO)4 by the addition and loss of CO (equation 7). If the cluster is formally unsaturated with respect to the electron counting mles, no change in the core structure takes place, for example, the interconversion of Fe4(CO)n(PPh)2 and Fe4(CO)i2(PPh)2 (equation 8). The base pair may be supplied by a conventional Lewis base (equation 7 or 8), but also can come from bond pairs, for example, H2 (equation 9). The latter is equivalent to oxidative addition in mononuclear... 

Formally unsaturated PN compounds arc called phosphazenes and contain P in the... 

However, the PN bonds in phosphazenes are considerably shorter than those observed in both amino derivatives and phosphorus pseudohalides, presumably because there is no actual alternating of single and double bonds in the formally unsaturated (NPX2) ring systems. These bonds are characterized by marked sensitivity to the influence of the substituents at the phosphorus atom ... 

The electrochemical generation and reactions of [PtL2l equivalents and related low valent, formally unsaturated intermediates, has been... 

Differential thermal analysis of the mixed-metal complex, cis-[Pt(SCN)(SCNAg)(NH3)2l[N03], which contains formally unsaturated silver(I), reveals an exothermic peak at 134-144 The process occurs... 

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