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A new approach to the synthesis in good yields of known cobalt and iron carbidecarbonyl clusters by CO cleavage in mild conditions is reported. Cleavage of CO results from attaching an acetyl or benzoyl carbocation to the oxygen atom, and by transfer of electrons from an external source. This synthetic approach to carbide molecular clusters may be of some significance with respect to the formation of carbide atoms on to metal crystallites. Attempts to synthesize nickel carbide clusters with the same approach have only been partly successful. The new [Ni9C(CO)17]2~ and [Ni8C(CO)13]2~ have been obtained more conveniently from the reaction of [Ni6(CO)12]2- with CC14. The related reaction of [Ni6(CO)12]2- with Co3(CO)9CCl results in the formation of the mixed-metal carbide cluster [Co3Ni9C(CO)20]3_. This compound is degraded under a carbon monoxide and hydrogen mixture (25 °C, 1 atm) to Ni(CO)4, [Co(CO)4]~ and ethane. Intermediate formation of [Co3Ni7(C-C) (CO)15]3-, in which the two carbide atoms show an interatomic separation of 1.43 A, or of a related species, would provide a possible pathway for C-C bond formation.

The reaction of [Ni 10 C 2 (CO) 15 ] 2- in thf with a large excess of CdCl 2 ·2.5H 2 O (8-15 equiv.) resulted in the formation of the new carbonyl octacarbide clusters [H 5-n Ni 36 C 8 (CO) 36 -(Cd 2 Cl 3 )] n- (n = 3-5), which undergo partial CO replacement to give [Ni 36-y C 8 (CO) 34-y (MeCN) 3 (Cd 2 Cl 3 )] 3- (y = 0-2) after a prolonged time in MeCN. Treatment of the former with an excess of NaOH afforded the larger [H 7-n Ni 42+y C 8 (CO) 44+y -(CdCl)] n- (n = 6, 7 ; y = 0, 1) octacarbides. Their structures (as well as those of the analogous Br-containing clusters) have been fully elucidated by single-crystal X-ray analysis of their [Me 4 N] 5 [Ni 36 C 8 (CO) 36 (Cd 2 Cl 3 )]·(7-2y)MeCN·yC 6 H 14 (y = 0.40), [Me 4 N] 3 [Ni 36-y C 8 (CO) 34-y (MeCN) 3 (Cd 2 Cl 3 )]·5MeCN (y = 0.61), [Me 4 N] 7 [Ni 42+y C 8 (CO) 44+y (CdCl)]·(5-y)MeCN (y = 0.19) and [Me 4 N] 6 [HNi 42+y C 8 (CO) 44+y (CdBr)]·(6-y)MeCN (y = 0.19) salts, which feature highly distorted metal cages (due to the inclusion of several carbide atoms), and the presence of partially vacant capping Ni(CO) fragments. This aspect, together with the fact that all these species undergo several protonation-deprotonation equilibria in solution as well as reversible redox processes under electrochemical control, indicates that a detailed description of molecular species containing a few dozen metal atoms might be sometimes rather troublesome and non-trivial. A complete elucidation of these systems can be achieved only by combining structural, chemical, spectroscopic, electrochemical and spectroelectrochemical studies.

A reinvestigation of the synthesis of [H(5-n)Rh(13)(CO)(24)](n-) (n = 2, 3) led to isolation of a series of Rh(19), Rh(26), and Rh(33) high-nuclearity carbonyl and carbonyl-substituted rhodium clusters. The [Rh(19)(CO)(31)](5-) (1) is electronically equivalent with [Pt(19)(CO)(22)](4-), but poor crystal diffraction data of all salts obtained to date have prevented its geometrical analysis. The structures of Rh(26)(CO)(29)(CH(3)CN)(11) (2) as 2·2CH(3)CN and [Rh(33)(CO)(47)](5-) (3) as [NEt(4)](5)[3]·Me(2)CO were determined from complete X-ray diffraction determinations. The latter two species adopt polyicosahedral metal frameworks, and notably, [Rh(33)(CO)(47)](5-) represents the molecular group 9 metal carbonyl cluster of highest nuclearity so far reported.

Four molecular Pt-carbonyl clusters decorated by Cd-Br fragments, i.e., [Pt(13)(CO)(12){Cd(5)(μ-Br)(5)Br(2)(dmf)(3)}(2)](2-) (1), [Pt(19)(CO)(17){Cd(5)(μ-Br)(5)Br(3)(Me(2)CO)(2)}{Cd(5)(μ-Br)(5)Br(Me(2)CO)(4)}](2-) (2), [H(2)Pt(26)(CO)(20)(CdBr)(12)](8-) (3) and [H(4)Pt(26)(CO)(20)(CdBr)(12)(PtBr)(x)](6-) (4) (x = 0-2), have been obtained from the reactions between [Pt(3n)(CO)(6n)](2-) (n = 2-6) and CdBr(2)·H(2)O in dmf at 120 °C. The structures of these molecular clusters with diameters of 1.5-2 nm have been determined by X-ray crystallography. Both 1 and 2 are composed of icosahedral or bis-icosahedral Pt-CO cores decorated on the surface by Cd-Br motifs, whereas 3 and 4 display a cubic close packed Pt(26)Cd(12) metal frame decorated by CO and Br ligands. An oversimplified and unifying approach to interpret the electron count of these surface decorated platinum carbonyl clusters is suggested, and extended to other low-valent organometallic clusters and Au-thiolate nanoclusters.

The reactions of [NEt(4)](2)[Ni(6)(CO)(12)] with miscellaneous carbon halides (e.g. CCl(4), C(4)Cl(6)) in CH(2)Cl(2) have been extensively investigated particularly focusing on the stoichiometric ratio of the reagents and reaction temperature. This allowed the preparation of the previously known acetylide clusters [Ni(16)(C(2))(2)(CO)(23)](4-), [HNi(25)(C(2))(4)(CO)(32)](3-) and [Ni(22)(C(2))(4)(CO)(28)Cl](3-), as well as isolation and full characterisation of the closely related [Ni(17)(C(2))(2)(CO)(24)](4-) and [Ni(25)(C(2))(4)(CO)(32)](4-) tetraanions. From a structural point of view, all these clusters are based on a Ni(16) square orthobicupola which contain interstitial C(2), Ni(η(2)-C(2))(4) or Ni(2)(μ-η(2)-C(2))(4) moieties, displaying rather short C-C bonds. Electrochemical studies reveal that all these species undergo different redox processes, even if their stability is rather limited. This is corroborated by an extensive analysis of the interaction between interstitial C(2) acetylide units and the metal cluster cage by Extended Huckel Molecular Orbital (EHMO) calculations, which indicates that tightly bonded C-C units are less effective than isolated C-atoms in stabilising the cluster cage.

The new [Pt-5(CO)(5){Cl2Sn(mu-OR)SnCl2}(3)](3-) (R = H, Me, Et, Pr-i; 1-4) clusters contain trigonal bipyramidal (TBP) Pt-5(CO)(5) cores, as certified by the X-ray structures of [Na(CH3CN)(5)][NBu4](2)[1]center dot 2CH(3)CN and [PPh4](3)[4]center dot 3CH(3)COCH(3). The TBP geometry, which is rare for group 10 metals, is supported by an unprecedented interpenetration with a nonbonded trigonal prism of tin atoms. By capping all the Pt-3 faces, the Sn(II) lone pairs account for both Sn-Pt and Pt-Pt bonding, as indicated by DFT and topological wave function studies. In the TBP interactions, the metals use their vacant s and p orbitals using the electrons provided by Sn atoms, hence mimicking the electronic picture of main group analogues, which obey the Wade's rule. Other metal TBP clusters with the same total electron count (TEC) of 72 are different because the skeletal bonding is largely contributed by d-d interactions (e.g., [Os-5(CO)(14)(PR3)(mu-H)(n)](n-2), n = 0, 1, 2). In 1-4, fully occupied d shells at the Pt-ax atoms exert a residual nucleophilicity toward the adjacent main group Sn(II) ions permitting their hypervalency through unsual metal donation.

The new [HFe(4)(CO)(12)](3-) cluster anion has been obtained in high yields by reduction of [Fe(4)(CO)(13)](2-) or [HFe(3)(CO)(11)](-) with a 6 M methylalcoholic KOH solution under a nitrogen atmosphere and isolated with miscellaneous tetrasubstituted ammonium salts. The [NEt(4)](3)[HFe(4)(CO)(12)] salt has been characterized by IR, (1)H and (13)C NMR, electrospray ionization mass spectrometry, and X-ray studies. Investigation of its protonation reaction afforded spectroscopic proof for the existence of its unstable isomeric [HFe(4)(CO)(11)(CO-H)](2-) and [H(2)Fe(4)(CO)(12)](2-) conjugated acids. The latter is probably isostructural with the [H(2)Ru(4)(CO)(12)](2-) congener. The nature of the first protonation product as a [HFe(4)(CO)(11)(CO-H)](2-) adduct, involving an oxygen-bound proton, has been corroborated by the preparation and spectroscopic characterization of the corresponding [HFe(4)(CO)(11)(CO-Me)](2-) dianion. The above findings demonstrate that protonation of a CO-shielded polynuclear metal anion initially occurs on one oxygen atom and then the oxygen-bound proton migrates to the metal cage. Finally, [HFe(4)(CO)(12)](3-) and its [H(2)Fe(4)(CO)(12)](2-) conjugate acid fill the previously existing gap between the chemistry of iron carbonyls and ruthenium and osmium congeners.

The reaction of [Ni9C(CO)17]2− with CdX2 · xH2O (X = Cl, Br, I) affords the tetra-carbide carbonyl clusters [H6−nNi30C4(CO)34(CdX)2]n− (n = 3–6; X = Cl, Br, I), which are transformed into the closely related [H7−nNi32C4(CO)36(CdX)]n− (n = 5, 6, 7; X = Cl, Br, I) by reaction with OH− or X− ions. A detailed crystal analysis clearly indicates that the latter is actually obtained in mixture with [H7−nNi31C4(CO)37(CdX)]n− and [H7−nNi33C4(CO)37(CdX)]n−, which differ only by the presence or the absence of Ni(CO) fragments. The above mixture can be considered to represent a fully characterized molecular example of “mono-dispersed colloids” of ligand-protected metal nanoparticles.

The inter-conversions of platinum carbonyl dianionic clusters, ([Pt(3)(CO)(6)](n)(2-), n = 2-5), have been studied in THF and acetonitrile using in situ FTIR spectroscopy. These inter-conversions were facilitated by the addition (or removal) of molecular hydrogen. The individual reactions, namely reductions and oxidations of [Pt(3)(CO)(6)](n)(2-) were fast and reversible. BTEM analysis of the data provided the pure component spectra of the individual species without the need for physical separation. It is shown, for the first time, that the species [Pt(3)(CO)(6)](n)(2-) (n = 2) can be formed from the reduction of [Pt(3)(CO)(6)](n)(2-) (n = 3-5) by hydrogen alone in acetonitrile. Also, detection of dissolved CO(2) in solution suggests that a room-temperature water gas shift reaction occurs. This has been shown to arise from nucleophilic attack of water on a coordinated CO of [Pt(3)(CO)(6)](n)(2-) which leads to the formation of [HPt(15)(CO)(19)](3-) and [H(2)Pt(15)(CO)(19)](2-). The parent tetraanion, [Pt(15)(CO)(19)](4-), has been isolated in high yields by reaction of [Pt(3)(CO)(6)](n)(2-) (n = 2, 3) with NaOH at 60 °C and has been structurally characterized by X-ray analysis.