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Rhodium complexes of formula [RhBp(CO)P] [Bp = bis(pyrazolylborate), P = P(NC4H4)3 1, PPh3 2, PCy3 3, P(C6H4OMe-4)3 4] have been prepared by exchange of the acetylacetonate (acac⁻) ligand in [Rh(acac)(CO)P] complexes. The spectroscopic and electrochemical properties as well as X-ray data of [Rh(acac)(CO)P] and [RhBp(CO)P] complexes have been compared with the aim to estimate the relative donor properties of both anionic ligands (acac⁻ and Bp⁻). The cyclic voltammetric results indicate that the Bp⁻ ligand behaves as a much stronger electron donor than acac⁻ and a value of the Lever EL ligand parameter identical to that of the pyrazolate ligand (−0.24 V vs. NHE for each coordinating arm) is proposed for the bis- and tris(pyrazolyl)borate ligands, whereas P(C6H4OMe-4)3 is also shown to have an identical EL value (0.69 V) to that of P(NC4H4)3. An improved linear relationship between the oxidation potential and the sum of the ligand EL values for square-planar Rh^I complexes is also obtained and adjusted values for the Lever SM and IM parameters for the Rh^I/Rh^II redox couple are given. The trans influence of phosphanes was not observed in crystals of complexes 2 and 3, in contrast to analogous acetylacetonato complexes in which the Rh−O bonds differ by ca. 0.04−0.06 Å. Complexes 1−4 are very attractive precursors for hydroformylation catalysts and yields of aldehydes of 80−87% have been obtained with all complexes without extra phosphane as co-catalyst. During the hydroformylation reaction, however, small amounts of a catalytically inactive [RhBp(CO)2] complex were formed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

A series of diorganotin(IV) and dichlorotin(IV) derivatives of 4-X-benzohydroxamic acids, [HL1 (X = Cl) or HL2 (X = OCH3)] formulated as [R2SnL2] (R = Me, Et, nBu, Ph or Cl; L = L1 or L2), along with their corresponding mixed-ligand complexes [R2Sn(L1)(L2)] have been prepared and characterized by FT-IR, ¹H, ¹³C, and ¹¹⁹Sn NMR spectroscopy, mass spectrometry, elemental analysis, and melting points. In addition, single-crystal X-ray diffraction analyses were carried out for [Me2SnL2] (L = L1 or L2), which show coordination structures intermediate between distorted octahedra and bicapped tetrahedra. The hydroxamate ligands are asymmetrically coordinated by the oxygen atoms, the carbonyl oxygen atom is further away from the metal center than the other oxygen atom. The complexes are stable monomeric species; most of them are soluble not only in chlorohydrocarbon solvents, but also in alcohols and hydroalcoholic solutions. In polar solvents, the mixed-ligand complexes gradually decompose into the corresponding single-ligand complex couples. The complexes exhibit in vitro antitumor activities (against a series of human tumor cell lines) which, in some cases, are identical to, or even higher than, that of cisplatin. For the dialkyltin complexes, the activity increases with the length of the carbon chain of the alkyl ligand and is higher in the case of the chloro-substituted benzohydroxamato ligand. The [nBu2Sn(L1)2] complex displays a high in vivo activity against H22 liver and BGC-823 gastric tumors, and has a relatively low toxicity.

In the di(organocyanamide) complexes trans-[PtCl₂(NCNR₂)₂] (R = Me 1a, Et 1b), prepared by reaction of PtCl₂ with the appropriate NCNR₂, the cyanamide ligands, activated by coordination, undergo an unprecedented nucleophilic addition, at room temperature, of an haloalcohol (ClCH₂CH₂OH/LiBun) or of an alcohol (MeOH) to give the corresponding 2-amino-2-oxazoline cis-[PtCl₂{&z.ub1s;N&z.dbd;C(OCH₂C&z.ub1e;H₂)NR₂}₂] (R = Me 2a, Et 2b) or trialkylisourea trans-[PtCl₂{NH&z.dbd;C(OMe)NR₂}₂] (R = Me 3a, Et 3b) complexes. The X-ray crystal structure of 2b is also reported, indicating a π-bond delocalization along the N&z.hbdda;C&z.hbdda;N group of the aminooxazoline ligands.

Cyanamide N&z.tbd;C–NH₂, on reaction with trans-[Mo(N₂)₂(dppe)₂] (dppe = Ph₂PCH₂CH₂PPh₂) in thf, undergoes reductive dehydrogenation to cyanoimide NCN2− with formation of trans-[Mo(NCN)₂(dppe)₂] 1 which is susceptible to protonation, alkylation, acylation, aroylation and silylation, on treatment with electrophiles (E+) such as HBF₄, [Et₃O][BF₄], RC(O)Cl (R = Et, Ph) or Me₃SiI, affording the corresponding addition products trans-[Mo(NCN)(NCNH)(dppe)₂][BF₄] 2, trans-[Mo(NCN)(NCNH₂)(dppe)₂][BF₄]₂ 3, trans-[Mo(NCN)(NCNEt)(dppe)₂][BF₄] 4, trans-[Mo(NCN)(NCNHEt)(dppe)₂][BF₄]₂ 7, trans-[Mo(NCN){NCNC(O)R}(dppe)₂]Cl (R = Et 5a, Ph 5b) and trans-[Mo(NCN)(NCNSiMe₃)(dppe)₂]I 6. The electrophilic addition to the exo-N atom is confirmed by the X-ray crystal structures of 2 and 4 which also show a linear multiple-bond coordination of the cyanoimide and the derived NCNE (E = H or Et) ligands, the latter behaving as weaker π-electron donors than the former.

The reaction of trans-[PtCl4(EtCN)2] and two equivalents of the formamides RR'NCHO (R/R' = Me/Me, Me/H, tBu/H) in CH2Cl2 led to the formation of the compounds cis-[PtCl4{(O=)CHNRR'}2], two of which (R/R' = Me/H, tBu/H) were characterized by X-ray single-crystal diffractometry, as well as by elemental analysis, FAB⁺-MS, and IR, ¹H, ¹³C and ¹⁹⁵Pt NMR spectroscopy (including NOE, HMQC and INEPT experiments). The X-ray structures and IR spectroscopic data indicate the coordination of the formamides through the O atom and significant contribution of the bipolar structure ⁻O−C(H)=⁺NHR in the resonance hybrid of the ligands. The complexes cis-[PtCl4{(O=)CHNRR'}2] represent the first example of (formamide)Pt^IV species and also a rare case of platinum(IV) complexes with neutral O-donor ligands.

Neutral alkenylcarbyne complexes [X = SCN; n = 1 (2a), 4 (2b). X = OCN; n = 1 (2c), 4 (2d)] were prepared by reaction of the acetonitrile complexes [n = 1 (1a), 4 (1b)] with M[OCN] or M[SCN] (M = Na, K, Bu4N), and were fully characterised. The synthesis of complex 3, containing the tridentate phosphane triphos [triphos = PPh(CH2CH2PPh2)2], is also described. The structures of 2b and 3 were determined by X-ray diffraction methods. In both complexes, the coordination of the W atom was octahedral, with the N-bonded thiocyanate (2b) or the chlorine (3) anions trans to the alkenylcarbyne group. The W−N and W−Cl bond lengths were significant longer than the average found in the literature; this indicates the remarkable trans influence of the alkenylcarbyne moiety. The electrochemical behaviour of complexes 2a−d and 3, as well as that of the related compounds D (2e: X = F, n = 4), (2f: X = Cl, n = 1), (2g: X = S2P(OEt)2, n = 1) and A (4: n = 1) was investigated by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in aprotic media and at a Pt electrode. They underwent irreversible anodic and cathodic processes, the former usually being multi-electronic in the time scale of CPE and involving anodically induced proton dissociation from the alkenylcarbynes. These ligands behave as remarkably strong π-electron acceptors and from the linear relationship between the oxidation potential of the first anodic wave and the electrochemical PL ligand parameter, it was possible to estimate, for the first time for a carbyne-containing metal centre, that is, the site {(dppe)(CO)2W≡C−R}, the values of the electrochemical electron richness (Es) and the polarisability (β) parameters. These values indicate that this site has low electron richness and low polarisability; this is accounted for by the extensive π-electron acceptance of the alkenylcarbyne ligand. PL was also estimated for the S2P(OEt)2⁻ ligand, which was shown to act as a rather strong net electron releaser.

Neutral trans-cyanide alkenylcarbyne complexes 2a and 2b have been prepared by reaction of the complex 1a and 1b with NaCN or [Bu4N]CN. The reaction of complexes 2a and 2b with an equimolar amount of the acetonitrile complexes 1a and 1b in CH2Cl2 leads to the cationic cyanide-bridged bis(alkenylcarbyne) di-tungsten complexes 3a–d. Diisocyanide-bridged bis(alkenylcarbyne) di-tungsten complexes 4a and 4b have been synthesized by the reaction of complexes 1a and 1b with 0.5 equivalents of the diisocyanide 1,4-(CN)2C6H4. IR as well as ¹H-, ³¹P{¹H}-, ¹³C{¹H}-, and ¹⁸³W-NMR data are reported. The spectroscopic data show that in the dinuclear complexes 3a–d, the bridging CN group and the alkenylcarbyne units are located in trans positions, while in the dinuclear complexes 4a and b, the isocyanide groups of the bridging ligand 1,4-(CN)2C6H4 and the two alkenylcarbyne moieties are cis. The ¹⁸³W chemical shifts of complexes 2a, 2b, 3a–d, 4a, and 4b were obtained through two-dimensional indirect ³¹P, ¹⁸³W NMR recording techniques. A downfield shifting of ¹⁸³W resonances of the cyanide-bridged dinuclear complexes 3a–d with respect to the mononuclear ones, 2a and 2b, was observed. The δ¹⁸³W of isocyanide bridging dinuclear complexes 4a and 4b appear at higher field than those of the corresponding mononuclear cyanide 2a and 2b in accordance with the higher π-acceptor electron properties of the isocyanide ligand. The electrochemical behaviour of all the complexes has been investigated by cyclic voltammetry and controlled potential electrolysis in aprotic media and at a Pt (or vitreous C) electrode. Complexes 1, 2, or 3 undergo multi-electron irreversible oxidation processes involving anodically induced proton dissociation from the alkenylcarbyne ligands, and irreversible cathodic processes are also observed for all the complexes. The order of the redox potentials reflects that of the net electron π-acceptor/σ-donor character of the ligands and the ligating alkenylcarbynes are shown to behave as remarkably strong π-electron acceptors (even stronger than CO).