Carbonyl Complexes of the Transition Metals

Abstract

Carbon monoxide is a versatile ligand as it forms compounds with both transition metals and main group elements. Transition d metals of electronic configuration d n (0 < n < 10) form the most numerous class of these compounds, although recent progress has been made in expanding the area of existence toward main group, lanthanide (4f) and actinide (5f) metals. The most extensively studied series of carbonyl derivatives have the general formula [M(CO) n ] m , with n representing the coordination number of the central metal atom M, and m being the nuclearity of the system. In these compounds, the central metal atom is in the zero oxidation state. Starting from readily available inorganic derivatives in a positive oxidation state, the synthesis of these compounds normally requires a reducing agent capable of acting in the presence of carbon monoxide: in fact, only nickel metal, as a finely divided activated powder, is reactive toward CO leading to the corresponding derivative Ni(CO) 4 . The selection of the reducing agent varies as a function of the position of the metal M within the periodic table. There is a general trend of an increase in the nuclearity m toward the end of the transition d series, the nuclearity being related to the bonding arrangements of the CO ligand, which may act as terminal, doubly bridging or triply bridging. Metal carbonyls are characterized by IR‐active vibrations around 2000 cm −1 associated with the carbonyl groups. Vibrational spectra in solution give important information about: (a) the electronic distribution within the M–CO bond in terminally bonded carbonyl groups; (b) the molecular structure associated with the number of bands observed; (c) the type of bonding in dinuclear or polynuclear compounds. X‐ray diffraction data have been extensively used in order to define the molecular structure in the solid state. The metal–CO combinations span a wide range of oxidation states, derivatives being known for oxidation states from +III to −IV. Metal carbonyls undergo three types of reaction: (a) substitution of the carbonyl groups, whereby the oxidation state of the central metal atom remains unchanged, (b) oxidation, and (c) reduction. Carbonyl derivatives of early‐ and late transition metals present specific bonding and spectroscopic properties. Concerning the transition elements of groups 6–9, the M–CO bond dissociation enthalpy increases down the group, and is normally greater than the metal–metal bond enthalpy, the latter being estimated from the heat of atomization. From a mechanistic viewpoint, substitution and redox reactions have been studied extensively. A special case of substitution is the migratory carbonyl insertion, by which an M–R bond to a carbyl group R is converted by CO into the M–(CO)R sequence.

Fluorine Migration from Carbon to Iron and Fluorine–Iron Dative Bonds in Octafluorocyclohexadiene Iron Carbonyl Chemistry

“…The rich area of metal carbonyl chemistry is of interest because of the possibility of the carbonyl ligands to stabilize low formal metal oxidation states . A prime example is chromium hexacarbonyl, Cr­(CO) 6 , which, despite the formally zerovalent chromium atom, is a fully air-stable white solid.…”

Extreme Metal Carbonyl Back Bonding in Cyclopentadienylthorium Carbonyls Generates Bridging C2O2 Ligands by Carbonyl Coupling

“…The first binary metal carbonyl to be synthesized was Ni(CO) 4 , reported in 1890 . During the next half century a number of other stable binary metal carbonyls were discovered including Fe(CO) 5 , Fe 2 (CO) 9 , Fe 3 (CO) 12 , Co 2 (CO) 8 , and M(CO) 6 (M = Cr, Mo, and W) . These are all commercially available and have become key intermediates for the synthesis of a variety of organometallic compounds.…”

Exploring the Intricacies of Weak Interactions in Metal–Metal Bonds Using an Unsymmetrical Carbonyl Precursor and a Triple-Bonded W26+ Paddlewheel

“…The bonding in the now ubiquitous transition metal carbonyls has been customarily described as a combination of two synergistic processes with one involving the formation of a σ bond (carbon → metal) derived from the interaction of the lone electron pair of the carbon atom in carbon monoxide with an unoccupied d orbitals in the metal unit. The second process is the back-bonding attained from the interaction of electrons in a filled metal d orbital with an empty π* MO of the carbonyl group . The stretching frequencies (ν CO ) have often been used to evaluate the strength of the carbonyl binding.…”