Cyclobutadiene-carbon monoxide complex

As was mentioned in Section III,A a very useful method of synthesis of olefin complexes involves the displacement of carbon monoxide from metal carbonyls by olefins. Under similar conditions acetylenes usually react to give new ligands, e.g., cyclobutadienes, cyclopentadienones, and quinones (see Sections V,E and VI,C), and it is not surprising, therefore, that the range of known acetylene complexes is smaller than the range of olefin complexes. 

Neutral (cyclobutadiene)iron complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate, and with the nitrosonium cation. Cationic nitrosyl complexes (e.g. 210) undergo ligand substitution by treatment with phosphines. Photolysis of (tetraphenylcyclobutadiene)Fe(CO)3 in THF at -40 °C is reported to give the novel bimetallic complex (214), which reacts with carbon monoxide (140 atm, 80 °C) to regenerate the starting material.An X-ray diffraction analysis of (214 R = Ph, R = t-Bu) reveals a very short Fe-Fe distance of 2.117 A. 

Photolysis of substituted (cyclobutadiene)Fe(CO)3 complexes in the presence of alkynes affords substituted benzenes (Scheme 60). From the substituent patterns of the products, it is clear that this reaction cannot be accounted for by generation of the free hgand. The proposed mechanism involves initial loss of a carbon monoxide ligand and coordination of the alkyne. Insertion of the alkyne predominantly into the... 

We found also that (77-C5H,)V(CO)3PPh3 as well as its niobium analog, when irradiated, underwent substitution of one PPh3 and one CO by tolan. Hence, niobium (and tantalum) complexes of the type are able to attach a third molecule of tolan and transform this to hexaphenylbenzene, unlike the vanadium analog, which transforms just two tolans to tetraphenyl-cyclobutadiene or, if irradiated, combines them with carbon monoxide and produces tetraphenylcyclone. 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

Neutral (cyclobutadiene)Co(CO)2X complexes and (cyclobutadiene)Co(CO)3+ cations undergo displacement in the presence of arenes to generate ( j -cyclobutadiene)(i -arene) Co+ cations (Scheme 69)38,2iie,228a jqeytraj (cyclobutadiene)MoCp(CO) complexes and (cyclobutadiene)RuCp(CO)+ cations also undergo ligand displacanent of a coordinated carbon monoxide. 

In M-generated metal carbonyl-complexed cyclopropenylphosphinidenes undergo a sequence of structural changes leading to phosphorus analogues of Pettit s seminal ( i(4)-cyclobutadiene)iron tricarbonyl complex via multiple valence isomers and the elimination of one molecule of carbon monoxide. ... 

The cobaltacyclopentadiene is converted on heating into a cyclobutadiene derivative. Treatment with carbon monoxide affords a cyclopentadienone complex. It is therefore probably an intermediate in the formation of these products from CpCo(CO) (L = CO) and alkynes. 

Irradiation of the cyclobutadiene complex Fe(CO)3(// -C4H4) results in the initial loss of a carbonyl ligand. Irradiation in a krypton matrix with X > 280 nm yields carbon monoxide and Fe(CO)2( / -C4H4) [Eq. (6.47)]. If the reaction is carried out at higher temperature, the bimetallic complex Fe2(CO)3( / -C4H4)2 is formed [Eq. (6.48)] > ... 

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