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Fausto Calderazzo
Calderazzo in 2004
Calderazzo in 2004
Born(1930-03-08)March 8, 1930
Parma, Italy
DiedJune 1, 2014(2014-06-01) (aged 84)
Pisa, Italy.
NationalityItalian
Education
Awards
  • A. Miolati award (1988)
  • L. Sacconi Medal (1998)
Scientific career
Fields
Institutions

Fausto Calderazzo (March 8, 1930. Parma – June 1, 2014. Pisa) was an Italian inorganic chemist. His academic career ranges from synthesis to mechanistic studies and he is considered a significant pioneer in the field of organometallic chemistry, especially in carbonyl chemistry[1]. His academic career ranges from synthesis to mechanistic studies. With the colleague Klaus Noack, he proved the mechanism of migratory insertion through the paradigmatic MnCH3(CO)5 carbonylation reaction.[2] Migratory insertion is a crucial reaction step in several homogenous catalytic cycles, mediated by metal transition complexes.

Biography[3]

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Early Life and Higher Education

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Fausto Calderazzo was born in Parma, on March 8th 1930, where his father served in the Royal Italian army. At the age of 17, at end of his high-school education, he was undecided whether to study chemistry or medicine. He finally decided on chemistry path, feeling it would be easier to find a job later. When he entered the University of Florence, in November 1947, the war had been over for two years, however it was still a very difficult time. University could provide only inexpensive chemicals and experiments were reduced to the essential. During the first day of the experimental thesis concerning an organic chemistry synthesis, Calderazzo was transferred to the laboratory of Luigi Sacconi[4] who was, at that time, Assistant Professor of Physical Chemistry at the University of Florence and was short of students. This circumstance changed his future scientific career. Professor Sacconi was not only a good teacher, but he also had an outstanding intuition for choosing a worthwhile topic for him. Calderazzo completed his research project for the Laurea degree on nickel hydrazide complexes in July 1952. Immediately after the degree he had to spend eighteen months as a conscript on military service.

Career

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After the compulsory military service, Calderazzo could resume his chemistry studies and join the research groupjoined the research group of Giulio Natta, Nobel laureate in 1963 with Karl Ziegler ("for their discoveries in the field of the chemistry and technology of high polymers"),[5] at the Institute of Industrial Chemistry in Polytechnic Institute of Milan. In this first employment he started to devote his work to the organometallic chemistry, and in particular, to the carbonyl derivates of transition metals.[6]

In Milan Calderazzo established a collaboration with Giulio Natta, Raffaele Ercoli, Piero Pino and Paolo Chini. Around the same time, the organometallic chemistry was the frontier of inorganic chemistry. In the same period Geoffrey Wilkinson at Imperial College of London and Ernst-Otto Fischer at the Technische Universität in Munich were exploring new synthetic pathways for low-valent air-sensitive materials.[7] During his professional life, Calderazzo had the opportunity to debate on carbonyl chemistry and to maintain personal relations with both of them. In 1959 Natta, Calderazzo et al. published the novel discovery of the first paramagnetic low-spin metal carbonyl: V(CO)6 (figure 1).[8] The discovery was the outcome in the attempting to find new metal-catalyzed routes to organic oxygenated compounds from available precursors and CO.

Figure 1. Crystallographic structure of V(CO)6.
  Oxygen
  Carbon
  Vanadium
A large static Jahn–Teller distortion is present.[9] Axial and equatorial V–C distances of 1.993 Å and 2.005 Å, respectively

In 1959, Fred Basolo (Northwestern University, Evanston, IL, USA) spending a sabbatical term in Italy, knew Calderazzo. They could discuss their similar and coincident studies. Basolo had a quite important influence on Calderazzo, therefore he began to focus on mechanistic studies with regard to migration reaction. Their scientific paths crossed on in other occasions and they continued to maintain contact during the following years.[10] In 1960 Calderazzo accepted the offer of being a Sloan Fellow with Frank Albert Cotton at Massachusetts Institute of Technology (MIT) in Cambridge, MA, USA.  He passed the postdoctoral year in a foreign laboratory, meeting colleagues with different backgrounds. About this experience, F. Albert Cotton said:

“We published a thorough study of the reaction involving CH3Mn(CO)5 and CH3COMn(CO)5, but the final answer to the mechanistic question was not found util some years later in Fausto’s own laboratory. I think Fausto’s solving of carbonyl insertion problem is a modern landmark in fundamental organotransition metal chemistry, with considerable relevance to industrial homogenous catalysis as well.”[11]

At the end of 1963 Calderazzo moved to Switzerland to join the Inorganic Synthesis Group at the Cyanamid European Research Institute, founded in 1959, in Geneva.[12] He was assigned Director of the group in 1965 and he held this position until 1968. In Geneva, using 13CO labeling, Calderazzo and Noack were able to demonstrate that the decisive carbon−carbon bond forming step, in carbonylation of MnCH3(CO)5, involves an insertion rather than simple migration, as had previously been supposed.[13] Migratory insertion takes part in several industrial catalytic cycles, and this specific experiment is used as a classical didactic model.[14][15] For instance, migratory insertion is considered to be a key-step in Roelen process for hydroformylation of ethylene[16] or methanol carbonylation,[17] catalyzed by [Co(CO)4]2 and [RhI2(CO)2]−, respectively. Other scientists such as Jack Halpern,[18] Wolfgang. A. Herrmann, Peter M. Maitlis, Malcom. L. H. Green, J. E. Ellis[19] and I. I. Moiseev began reading and debating his work.[20] The experience at the Cyanamid European Research Institute made Calderazzo conscious of the important linkage between research and industry. He learned how stringent the conditions imposed by industries can be, in terms of cost and productivity of a process. Calderazzo returned to Italy as Full Professor at the Institute of General and Inorganic Chemistry of the University of Pisa in 1968. He also taught advanced courses of inorganic chemistry at the Scuola Normale Superiore of Pisa. His arrival in town was concomitant with the political and social turmoil of Youth Protests and he had to face some student unrests. Despite this, Calderazzo set up a research team with numerous collaborators, including Carlo Floriani, Giuseppe Fachinetti, Dario Vitali, Daniela Belli, Marco Pasquali, Fabio Marchetti, Guido Pampaloni and Luca Labella. The Pisan time represents the longest and most worthwhile section of Calderazzo’s scientific carrier. In Pisa he carried on the experimental activity, funded by American and European industries (particularly ENI), in parallel with teaching role.Teaching, especially to young students, was always a very enjoyable part of his job. About this he said:

“Young people should realize that Chemistry is a fascinating part of the Science; how atoms combine to make molecules and extended aggregates, how thermodynamics govern the combination of these atoms is something that still require further investigation” [21]

During his scientific activity, Calderazzo extended his interest in carbonyl chemistry of the late transition metals like Pd, Pt[22] and Au (figure 2)[23] in addition to the investigations in the field of, so called, “non classical” carbonyl compounds.[24][25]

Figure 2. Crystallographic structure of the mixed valence gold chloride Au4Cl8.
  Gold
  Chlorine

Other contributions from the work in Pisa are studies around derivates of early transition metals, which expanded and clarified the knowledge on this area of Periodic Table.[26] [27] Since middle 1970s Calderazzo carried out research projects with his collaborators, involving carbon dioxide as a C1 synthon for organic synthesis. They synthesized N,N-dialkylcarbamato complexes of elements, from transition to lanthanide metals, using secondary amines, CO2 and suitable precursors.[28][29]

Calderazzo died in Pisa, on June 1, 2014, at the age of 84.[30]

Awards and Titles

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Calderazzo was a member of the editorial or advisor board of international scientific journals such as Organometallics, Inorganic Chemistry, Dalton Transactions and Journal of Organometallic Chemistry. Furthermore, he was a member of international conference committees like the International Conferences on Organometallic Chemistry ICOMC, member of the Société Royale de Chemie (1987), Società Chimica Italiana and Accademia Nazionale dei Lincei (1989). He received the A. Miolati award in Inorganic Chemistry in 1988 and the L. Sacconi Medal in 1998.

Works

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On Migratory Insertion

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See also: Migratory insertion

The equilibrium and the rates of the forward and reverse reactions in the system were studied at atmospheric pressure of CO in various solvents and at several temperatures by Calderazzo and Cotton in 1961.[31] The reaction proved to be first order in both MnCH3(CO)5 and CO and the effect of changing the solvent appeared to be mainly a function of the dielectric constant in stabilizing a somewhat polar transition state. The volume of CO absorbed or evolved at a given time was determined by gas-volumetric measurements. A scheme of the burette used for the experiments is shown in the figure 3. Once the volume of CO was measured, it was converted to moles of CO and the CO/Mn mole ratio was determined. By measuring the amount of CO absorbed as a function of time, it was possible to study the kinetics of the carbonylation reaction.

Figure 3. Gas-volumetric burette: A marks the 100 mL burette; B the Hg containing bulb; C the capillary tube connected to atmosphere; D the three-way stopcock (CO flux); E the 50 mL two-neck Erlenmayer flask (reaction system). Liquid Hg is colored Red and the Yellow meniscus is the same solvent contained in Erlenmayer flask. The apparatus is thermostatically controlled.

In 1967 Calderazzo and Noack re-examined the prototype system using 13CO, in gas phase 50% enriched.[32] The reaction product was investigated by infrared spectroscopy, monitoring changes in terminal CO stretching bands. The compound MnL(CO)5, where L can be a methyl or acetyl group, has three stretching active IR vibrations due to C4v molecular symmetry.[33] The key signals were the axial 12CO and 13CO stretching bands (trans to L), respectively at 1991 cm-1 and 1949 cm-1. Compared to 12CO, the 13-labelled molecule shows a C-O bond stretching absorption shifted to lower wavenumbers. First of all, it was observed that the incoming 13CO have not been inserted in between the metal carbon bond Mn-CH3. They obtained a 100% cis isomer of [Mn(COCH3)(13CO)(CO)4] (scheme 1).

Scheme 1. Note the solvent changing in the backward reaction: heptane is a high-boiling solvent, suitable for the temperature indicated.

Two hypothetical mechanisms had been previously proposed: the CO or CH3 intramolecular migration. Calderazzo and Noack designed an experiment based on the Principle of Microscopic Reversibility:[34] in mechanistic terms, if a certain series of steps constitutes the mechanism of a reverse reaction, the mechanism of the forward reaction, under the same conditions, is given by the same steps traversed forwards. They examined the decarbonylation of acetyl complex, prepared either by about 50% enriched [MnCH3(13CO)(CO)4] or prepared from CH3(13CO)CI and Na[Mn(CO)5]. They found a mixture of cis and trans product [MnCH3(CO)4(13CO)], 50% and 25% respectively (in ratio of 2:1). They obtained also 25% of unlabelled CH3Mn(CO)5 (scheme 2). According to the Principle of Microscopic Reversibility, this result confirmed the intramolecular CH3 migration hypothesis, because the carbonyl migration process should have shown only the cis product [MnCH3(13CO)(CO)4] with a probability of 75% and the free 13CO product with 25% .

Scheme 2. Methyl migration.

Successive researches revealed also that cis-regiospecific alkyl or aryl migrations occur due to addition of neutral ligands (2 electron donor),[35] and that a penta-coordinate intermediate, involved in the intramolecular mechanism, allows the retention of configuration (scheme 3). The kinetics are reminiscent of dissociative substitution except that the vacant site is formed at the metal during the migratory step, not by loss of a ligand. Using the steady-state method, the rate is given:

It was reported that the rates of the carbonylation (and the decarbonylation) reaction are dependent on the alkyl group R. Plotting the rate constats versus the number of carbon atoms in the alkyl chain for the reaction of [Mn(R)(CO)5] with PPh3, it is confirmed an increase from methyl to n-propyl derivate, followed by a slowly decrease.[36] Further experiments revealed that, not only Lewis acids are able to increase the reaction rate activating the carbonyl group, but also stabilizing the product and transition state. Other factors accelerating the reaction are polar, electron-withdrawing containing solvent, and bulky ligands Ln.[37][38]

Scheme 3. Kinetic constants K(n) are colored orange.

Manganese(I) Alkyl Carbonyl Complexes

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The involvement of migratory insertion in catalytic cycles of large industrial interest has contributed to maintain continuous interest in this mechanism.[39] For instance, recently it has been highlighted the potential of alkyl Mn(I) carbonyl-based homogenous catalysts. New investigations have explored the catalyzed activation under milder conditions of nonpolar or moderately polar bond such as H−H, B-H, C-H and Si-H.[40] Chemists have taken advantage of the possibility that Mn(I) alkyl complexes undergo migratory insertion, yielding an unsaturated acyl intermediate. In fact, hydrogen atom abstraction by the acyl ligand  paves the way to an active species for a variety of fast and reversable transformations, proceeding via an inner-sphere process. A remarkable example is the dehydrogenative silylation of alkenes (scheme 4).

Scheme 4. Dehydrogenative Silylation of Styrene Yielding to the corresponding 96% Allyl Silane ([Si]= SiEt3) catalyzed by Mn complex (R= iso-propyl). The reaction is diastereoselective with E/Z> 99:1.

On Early Transition Metals

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The majority of neutral homoleptic transition metals carbonyls obeys the eighteen electrons rule and basically is low spin. In fact carbon monoxide is a strong field ligand and produces large splitting between π occupied and non-bonding eg orbitals. When in 1959 Giulio Natta presented to the Accademia Nazionale dei Lincei the discovery of the first paramagnetic metal carbonyl (17 electrons species),[41][42] the announcement was followed by a serious scientific controversy. Virtually at the same time, Pruett and Wyman (Union Carbide, West Virginia, USA) obtained vanadium carbonyl by a different route. They suggested the formation of a diamagnetic, binuclear species V2(CO)12 with Vanadium-Vanadium bond.[43] By magnetic-susceptibility measurements Calderazzo and coworkers showed that vanadium carbonyl is a mononuclear, paramagnetic compound with a value for the effective magnetic moment, µeff, close to the spin-only value.[44] They suggested a ground state 2T2g for the open-shell d5 configuration.

In 1967 Pratt and Myers proved that, at liquid-helium temperatures, either a tetragonal or trigonal distortion is present.[45] Some years later, Bernier and Kahn indicated that between 300 and 66 K, V(CO)6 has roughly an octahedral geometry but that a dynamic Jahn-Teller effect is present, and further under 66K solid V(CO)6 is antiferromagnetic.[46] Finally in 1979 X-Ray diffraction analysis confirmed the monomeric structure of V(CO)6 and the Jahn-Teller distortion[47]. The 17-electron configuration confers to V(CO)6 unique properties with respect of comparable neutral homoleptic hexacarbonyls: V(CO)6 is stable only under inert atmosphere and shows high reactivity. It undergoes ligand substitution with Associative Mechanism (scheme 5).[48] Moreover, it is readily reduced to the 18-electron hexacarbonylvanadate(-1).

Scheme 5. CO substitution with phosphine goes by associative pathway: the kinetic equation shows the dependence from both phosphine and V(CO)6 concentration; the change in entropy of activation is negative.

The synthesis of V(CO)6 started from (acetyl acetonate)vanadium or VCl3 which was reacted with CO, under high pressure and temperature (135 atm and 135 oC), in the presence of pyridine and an electropositive metal like sodium to give sodium hexacarbonyl vanadate(-1), Na[V(CO)6]. The sodium salt  was oxidized by HCl in ether. In 1982, following up the research on hexacarbonyl metalates(−I) of group 5, Calderazzo and G. Pampaloni optimized the reaction conditions.[49] The team developed also a new process for the synthesis of V(CO)6, consisting of a one-electron oxidation of Na[V(CO)6] with HCl in pentane at −70 °C. All their efforts to isolate and characterize the Tantalum and Niobium (-1) neutral hexacarbonyls failed. However, they verified that under the same experimental conditions, Na[Nb(CO)6] and Na[Ta(CO6)] underwent two-electron oxidations to the halo-carbonyls of the metal, in the oxidation state (+1), (figure 4).[50][51]

Figure 4. Crystallographic structure of [H(THF)2][Nb2(µ-Cl)3(CO)8]:
  Oxygen
  Carbon
  Chlorine
  Niobium
  Hydrogen (only one is showed)

In addition, their work led to the synthesis of Nb and Ta(0) carbonyl complexes, stabilized by mixed metal clusters containing Silver (see figure 5 for Tantalum species).[52] Much later, in 2020, neutral homoleptic M2(CO)12 (M= Nb, figure 6, Ta) was finally isolated by I. Krossing and co-workers.[53] They obtained stable crystals at room temperature for several hours, under CO pressure in pentane solution. The tantalum carbonyl, Ta2(CO)12, was prepared by direct oxidation of [Ta(CO)6]- with Ag+ and the displacement of one CO ligand of [Ta(CO)7]+ by another equivalent of [Ta(CO)6]−.

Figure 5. Chemical diagram of Ag3Ta3(CO)12(Me2PCH2CH2PMe2)3
Figure 6. Crystallographic structure of Nb2(CO)12:
  Oxygen
  Carbon
  Niobium

Descending the group 5, it is evident how Nb and Ta obey more strictly to eighteen electrons rule and they are not stable, under normal conditions, as neutral homoleptic hexacarbonyls. On the other hand, V(CO)6 is  too sterically crowded to dimerize.

Arene Derivatives of Early Transition Metals[54]

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In 1964, during his period at the European Cyanamid Research Institute, Calderazzo obtained the V(η6-1,3,5-C6H3Me3)2 by the reaction of VCl3 with Al/AlCl3 in 1,3,5-C6H3Me3 followed by hydrolysis in alkaline medium. However, the procedure led to product in low yield.[55] Revisiting this work many years later, Calderazzo and colleagues performed a one-pot and high yield synthesis of V(η6-1,3,5-C6H3Me3)2, thanks to the change of the polarity of the medium and a large excess of Aluminum used as reducing agent. The same procedure was also applied to the synthesis of the bis-mesitylene derivatives of Nb[56], Cr, Mo(0) and bis-arene of Ti(0).[57] The method and its variants were patented as well as the use of V(η6-1,3,5-C6H3Me3)2 for preparing solid solutions of vanadium and titanium halides VTi3Cl12. The collaboration with ENI succeeded in producing V(η6-1,3,5-C6H3Me3)2 in almost quantitative yield from VCl3 on a kilogram scale. The chemistry of these arene early metal derivates revealed to be a useful synthetic tool to access conspicuous derivates in non-aqueous systems (scheme 6).[58] The driving force of this reactivity is the tendency toward high oxidation states attainment, for highly electron-positive metals such as group 5 metals.

Scheme 6. Reactivity of V(η6-mes)2 (mes= 1,3,5-C6H3Me3). Crystallographic structures (Hydrogens are omitted for simplicity)
  •   Vanadium
  •   Oxygen
  •   Carbon
  •   Flourine
  •   Iodide
  •   Aluminium
  •   Chlorine

Furthermore, Calderazzo and coworkers presented the first example of a tetra-arylborate anion (behaving as a 12-electron donor) obtained by thermal decomposition of Niobium(I) bis-arene derivative.[59] In the field of group 4 metal derivates, F. Calderazzo and co-workers studied the treatment of a toluene solution of Ti(η6 -1,3,5-iPr3C6H3)2 , as obtained in cooperation with M. L. H. Green of Oxford University, with [FeCp2]BAr4 which afforded the orange, paramagnetic cation [Ti(η6 -1,3,5-iPr3C6H3)2]+ (figure 7),[60] representing the first fully characterized titanium(I) derivative.

Figure 7. Crystallographic structures of Nb(η2-C2H2)(η6-C6H4F)2B(C6H4F)2 and [Ti(η6 -1,3,5-iPr3C6H3)2][(p-C6H4F)4] (Hydrogens are omitted for simplicity):
  •   Carbon
  •   Flourine
  •   Borum
  •   Titanium
  •   Niobium

On Lanthanides

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The lanthanide contraction was originally recognized and described on the basis of X-ray crystallographic determinations on the oxides and fluorides of the Ln(III) ions, which are not isostructural along the whole series.[61] In 1999 Calderazzo et al. showed the trend of the Ln-Oxygen distances as a function of the electronic configuration of the neutral lanthanide carbamate, with the same types of ligand and the same coordination number and geometry.[62] They prepared tetranuclear isotypic N,N-dialkylcarbamato complexes [Ln4(O2CNR2)12], following the procedure:

The mean values of the distances for each coordination bond type were plotted against the number of f electrons of the metal cation Ln(III). The di-isopropyl derivatives, with four of the major coordination modes are shown qualitatively in figure 8. Along the series, curves have rather similar slopes, suggesting that the metal radius contraction should mainly account for the trend and that early lanthanides contract their radii slightly more rapidly than the later ones. The N,N-diisopropyl derivatives from Neodynium to Lutetium are isotypic species, consisting of tetranuclear molecules, [Ln4(O2CNiPr2)12], where the metal center is hepta-coordinated.

Figure 8. Polynomial fittings of Ln-Oxygen distances as a function of 4f electronic configuration.

References

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  4. ^ . Bertini. “Luigi Sacconi february 1911–september 1992”. Coord. Chem. Rev., 1992, 120, IX-XII. https://doi.org/10.1016/0010-8545(92)80044-R.
  5. ^ Award ceremony speech. NobelPrize.org. Nobel Prize Outreach AB 2024. Thu. 25 Jan 2024. https://www.nobelprize.org/prizes/chemistry/1963/ceremony-speech/
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  14. ^ R.H. Crabtree, The organometallic chemistry of the transition metals. Wiley, New York (2005).
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  20. ^ see note 1.
  21. ^ see note 3.
  22. ^ D. B. Dell'Amico, F. Calderazzo, F. Marchetti, S. Ramello. “Molecular Structure of [Pd6Cl12] in Single Crystals Chemically Grown at Room Temperature”. Angew. Chem., 1996, 35, 1331-1333. https://doi.org/10.1002/anie.199613311.
  23. ^ D. B. Dell'Amico, F. Calderazzo, F. Marchetti, S. Merlino, G. Perego. “X-Ray crystal and molecular structure of Au4Cl8, the product of the reduction of Au2Cl6 by Au(CO)Cl”. J. Chem. Soc., Chem. Commun., 1977, 31-32. DOI: https://doi.org/10.1039/C39770000031.
  24. ^ D. B. Dell'Amico, F. Calderazzo, P. Robino, A. Segre. “Halogenocarbonyl complexes of gold”. J. Chem. Soc., Dalton Trans., 1991, 3017–3020. https://doi.org/10.1039/DT9910003017.
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  26. ^ F. Calderazzo, U. Englert, A. Guarini, F. Marchetti, G. Pampaloni, A. Segre. “[Zr3Cp2(O2CNiZPr2)6(µ3-O)(µ2-CCO)], the First Crystallographically Established Ketenylidene Complex; A Model for CO Reductive Cleavage on Metal Surfaces”. Angew. Chem., 1994, 33, 1188-1189. https://doi.org/10.1002/anie.199411881.
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  32. ^ See note 13.
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  34. ^ R. L. Burwell Jr., R. G. Pearson. “The Principle of Microscopic Reversibility”. Phys. Chem., 1966, 70, 300–302. https://doi.org/10.1021/j100873a508.
  35. ^ K. Noack, M. Ruch, F. Calderazzo. “The Mechanism of Reaction of Methylmanganese Pentacarbonyl and Acetylmanganese Pentacarbonyl with Triphenylphosphine”. Inorg. Chem., 1968, 7, 345–349. https://doi.org/10.1021/ic50060a037.
  36. ^ M. Andersen, J. R. Moss. “Synthesis of an Extensive Series of Manganese Pentacarbonyl Alkyl and Acyl Compounds: Carbonylation and Decarbonylation Studies on [Mn(R)(CO)5] and [Mn(COR)(CO)5]”. Organometallics, 1994, 13, 5013–5020. https://doi.org/10.1021/om00024a051.
  37. ^ E. J. Kuhlmann, J. J. Alexander. “Carbon monoxide insertion into transition metal-carbon sigma-bonds”. Coord. Chem. Rev., 1980, 33, 195-225. https://doi.org/10.1016/S0010-8545(00)80454-3.
  38. ^ F. Calderazzo. “Synthetic and Mechanistic Aspects of Inorganic Insertion Reactions. Insertion of Carbon Monoxide”. Angew. Chem. 1977, 165, 299-311. https://doi.org/10.1002/anie.197702991.
  39. ^ M. T. Whited, G. E. Hofmeister. “Synthesis and Migratory-Insertion Reactivity of CpMo(CO)3(CH3): Small-Scale Organometallic Preparations Utilizing Modern Glovebox Techniques”. J. Chem. Educ. 2014, 91, 1050–1053. https://doi.org/10.1021/ed500114a.
  40. ^ S. Weber, K. Kirchner. “Manganese Alkyl Carbonyl Complexes: From Iconic Stoichiometric Textbook Reactions to Catalytic Applications”. Acc. Chem. Res. 2022, 55, 2740−2751. https://doi.org/10.1021/acs.accounts.2c00470.
  41. ^ G. Natta, R. Ercoli, F. Calderazzo, A. Alberola, P. Corradini, G. Allegra, Atti. Acad. Naz. Lincei, Rend. Classe Sci. Mat. Nat., 1959, 27, 107. https://www.lincei.it/it.
  42. ^ See note 8.
  43. ^ R. Pruett, L. Wyman. Chem. Ind. (London)., 1960, 119-120.
  44. ^ F. Calderazzo, R. Cini, P. Corradini, R. Ercoli, G. Natta. Chem. Ind. (London)., 1960, 500 and 934.
  45. ^ [1] D. W. Pratt, R. J. Myers. “Electron spin resonance spectrum of vanadium hexacarbonyl at liquid-helium temperatures”. J. Am. Chem. Soc., 1967, 89, 6470–6472. https://doi.org/10.1021/ja01001a016
  46. ^ J.C. Bernier, O. Kahn. “Magnetic Behaviour of vanadium hexacarbonyl”. Chem. Phys. Letters, 1973, 19, 414. https://doi.org/10.1016/0009-2614(73)80394-X.
  47. ^ S. Bellard, A. K. Rubinson; M. G. Sheldrick. "Crystal and Molecular Structure of Vanadium Hexacarbonyl". Acta Crystallographica. 1979, B35, 271–274. https://doi.org/10.1107/S0567740879003332.
  48. ^ F. Basolo. “Kinetics and mechanisms of CO substitution of metal carbonyls”. Polyhedron, 1990, 9, 1503-1535.https://doi.org/10.1016/S0277-5387(00)86570-5.
  49. ^ F. Calderazzo, G. Pampaloni, G. Pelizzi. “The hexacarbonylniobate(−I) anion”. J. Organomet. Chem. 1982, 233, C41-C43. https://doi.org/10.1016/S0022-328X(00)85584-9.
  50. ^ F. Calderazzo, M. Castellani, G. Pampaloni, P. F. Zanazzi. “New carbonyl derivatives of niobium(I) and tantalum(I). Journal of the Chemical Society”. J. Chem. Soc., Dalton Trans., 1985, 1989-1995.[4]
  51. ^ F. Calderazzo, G. Pampaloni. “Metal carbonyl complexes of group VB metals”. J. Organomet. Chem., 1986, 303, 111-120. https://doi.org/10.1016/0022-328X(86)80116-4.
  52. ^ F. Calderazzo, G. Pampaloni, U. Englert, J. Strähle. “Electron-transfer processes with substituted group 5 metal carbonyls. Synthesis, crystal and molecular structure of Ag3M3(CO)12(Me2PCH2CH2PMe2)3, M = Nb, Ta, the first structurally characterized carbonyl derivatives of niobium(0) and tantalum(0)”. J. Organomet. Chem., 1990, 383, 45-57. https://doi.org/10.1016/0022-328X(90)85121-E.
  53. ^ W. Unkrig, M. Schmitt, D. Kratzert, D. Himmel, I. Krossing. “Synthesis and characterization of crystalline niobium and tantalum carbonyl complexes at room temperature”. Nat. Chem., 2020, 12, 647–653. https://doi.org/10.1038/s41557-020-0487-3.
  54. ^ G. Pampaloni. “Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity, and the applications of bis(η6-arene) complexes”. Coord. Chem. Rev., 2010, 254, 402-419. https://doi.org/10.1016/j.ccr.2009.05.014.
  55. ^ F. Calderazzo. “The Dimesitylenevanadium(I) Cation”. Inorg. Chem., 1964, 3, 810–814. https://doi.org/10.1021/ic50016a006.
  56. ^ F. Calderazzo, G. Pampaloni, L. Rocchi, J. Strähle, K. Wurst. “Synthesis and reactivity of η6-arene derivatives of niobium(II), niobium(I), and niobium(0)”. J. Organomet. Chem., 1991, 413, 91-109. https://doi.org/10.1016/0022-328X(91)80041-H.
  57. ^ F. Calderazzo, U. Englert, G. Pampaloni, M. Volpe. “Redox reactions with bis(η6-arene) derivatives of early transition metals”. J. Organomet. Chem. ,2005, 690, 14, 3321–3332. https://doi.org/10.1016/j.jorganchem.2005.03.064.
  58. ^ F. Calderazzo, G. E. De Benedetto, G. Pampaloni, C. Maichle-Mössmer, J. Strähle, K. Wurst. “Bis(arene)vanadium(0) complexes as a source of Vanadium(II) derivatives by both disproportionation of the [V(η6-arene)2]+ cations and oxidation of [V(η6-arene)2]”. J. Organomet. Chem., 1993, 451, 73-81. https://doi.org/10.1016/0022-328X(93)83010-S.
  59. ^ F. Calderazzo, G. Pampaloni, L. Rocchi, U. Englert. “Synthesis, Reactivity, and Crystal and Molecular Structures of Nb(L)(η6-C6H5-nXn)2B(C6H5-nXn)2, a Class of Mononuclear Metal Compounds Containing the 12-Electron-Donor Tetraarylborato Ligand”. Organometallics, 1994, 13, 2592–2601. https://doi.org/10.1021/om00019a016.
  60. ^ [1] F. Calderazzo, I. Ferri, G. Pampaloni, U. Englert, M. L. H. Green. “Synthesis of [Ti(η6-1,3,5-iPr3C6H3)2][BAr4] (Ar = C6H5, p-C6H4F, 3,5-(CF3)2C6H3), the First Titanium(I) Derivative”. Organometallics, 1997, 16, 3100–3101. https://doi.org/10.1021/om970155o.
  61. ^ D. H. Templeton,C. H. Dauben. “Lattice Parameters of Some Rare Earth Compounds and a Set of Crystal Radii”. J. Am. Chem. Soc., 1954, 76, 5237–5239. https://doi.org/10.1021/ja01649a087.
  62. ^ U. Abram, D. B. Dell’Amico, F. Calderazzo, C. Della Porta, A. Merigo, U. Englert, F. Marchetti. “Isotypical N,N-dialkylcarbamato lanthanide complexes covering a range of 11 atomic numbers: direct experimental assessment of the lanthanide contraction in trivalent molecular compounds”. Chem. Commun., 1999, 2053-2054. DOI: https://doi.org/10.1039/A904792A.
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