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INTRODUCTION


Transition Metal s can form complexes with Carbon Monoxide as a Ligand . These may be Homoleptic complexes such as Nickel Carbonyl (Ni(CO)4)or may contain a variety of ligands such as Re(CO)3(2,2'-bpy)Cl.

Carbon monoxide bonds to transition metals using synergic pi back-bonding.

The full sigma bonding orbital on the carbon monoxide overlaps with an empty d Orbital . A full d orbital overlaps with a pi antibonding orbital on the carbon monoxide which has the effect of weakening the carbon-oxgyen bond compared with free carbon monoxide.

''HOMOLEPTIC'' CARBONYLS



Overview


They are generally stable, easy available, and toxic. The forumla of many Homoleptic metal carbonyls can easily be inferred from the 18 Electron Rule . One of the easy ways to remember the order of the late first row Transition Metal s is to memerise the formula and strucutres of the metal carbonyls between Chromium and Nickel .

  • Group 4 Element s and Group 5 Element s with 4 and 5 valence electrons, because of steric effects it would be difficult to pack the required number of carbonyls arround the metal; centre to obtain an 18 VE complex. Hence ''homoleptic'' carbonyl complexes of these metals are rare. It is known that it is possible to obtain at low tempertures a 17 VE Vanadium carbonyl {V(CO)6}.

  • Group 6 Element s with 6 valence electrons form metal carbonyls Cr(CO)6 , Mo(CO)6 and W(CO)6 (6 + 6x2 = 18 electrons).

  • Group 7 Element s with 7 valence electrons form metal carbonyl dimers Mn2(CO)10, Tc2(CO)10 and Re2(CO)10 (7 + 1 + 5x2 = 18 electrons).

  • Group 8 Element s with 8 valence electrons form metal carbonyls Fe(CO)5 ,Ru(CO)5 and Os(CO)5 (8 + 5x2 = 18 electrons).

  • Group 9 Element s with 9 valence electrons form metal carbonyl dimers Co2(CO)8 and tetramers Co4(CO)12, Rh4(CO)12 and Ir2(CO)10 (9 + 3 + 3x2 = 18 electrons). Co2(CO)8 unlike the majority of the other 18 VE transition metal carbonyls is very sensitive to oxygen. Co2(CO)8 is used in Organic Chemistry ; it reacts with acetylenes ( Alkynes ) to form complexes, which are then used in the synthesis of organic compounds.

  • Group 10 Element s with 10 valence electrons form metal carbonyls Ni(CO)4 (10 + 4x2 = 18 electrons).


  • antibonding Carbonyl orbital. CO is therefore a Π acceptor and stabilizes electron-rich low valent metals.



Synthesis


Nickel Carbonyl and Iron Pentacarbonyl can be formed by the reaction of Carbon Monoxide with the metals, the other ''homoleptic'' carbonyls are normally made by the reduction of a high velent metal compound in an Autoclave with Carbon Monoxide present.


HETEROLEPTIC CARBONYLS


These are important in chemistry as carbonyl ligands can oftein be found with other ligands attached to a metal. For instance Vaska's Complex and Carboxyhemoglobin are important complexes in chemistry and Biology .

In addition to the ''homoleptic'' carbonyl complexes the halo pentacarbonyls of Manganese are very important in Organometallic chemistry. Many important compounds such as are made from [MnCl(CO)5 (also in addition to the Chloride the Bromide is also known).

An important synthesis route for Ruthenium , Osmium , Rhodium and Iridium carbonyls is the reaction of either DMF or a high boiling point Alcohol (such as methoxyethanol) with the metal chloride. The carbonyl is formed from the solvent, much of the early work on this topic was done by Lauri Vaska .


THIOCARBONYLS


Due to the instability of Carbon monosulfide the thiocarbonyl complexes are formed reactions which convert other ligands into the thiocarbonyl ligand.


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