The bonding in the triruthenium dihydrido cluster compound [Ru 3 (μ-H) 2 (μ 3 -κ 2 -MeImCH)(CO) 9 ] (1), which contains a face-capping N-heterocyclic carbene ligand (MeImCH) derived from the activation of two C-H bonds of 1,3-dimethylimidazol-2-ylidene (Me 2 Im), has been studied from the perspective of the atoms in molecules (AIM) quantum theory. Although the AIM approach recognizes the existence of a bond path in only one of the Ru-Ru edges of complex 1, i.e., that unbridged by the hydride ligands Ru(1)-Ru(3), the non-negligible values for the delocalization indexes of the hydride-bridged Ru-Ru edges indicate a delocalized kind of metal-metal interaction in these edges. In fact, a multicenter (5c-6e) interaction involving the Ru 3 H 2 core of the molecule can be proposed. The three-atom C-N-C bridge that spans the Ru(1)-Ru(3) edge of 1 does not delocalize the electronic density of the bridged metal atoms as efficiently as bridges comprising just one atom, such as hydride or CH. The topological parameters of the three Ru-C bonds between the metal atoms and the face-capping NHC ligand are very similar, and they confirm that these interactions are pure σ-bonds. The analysis of the topological parameters for the bonds of the NHC ligand confirms the presence of π-electron delocalization within the five-membered ring as well as the existence of some double-bond character in the interaction of the carbene C atom with the adjacent N atoms.

The quantum theory of atoms in molecules (QTAIM) has been applied to the recently synthesized alkaline-earth cubic O h -symmetric complexes Ca­(CO)8 (1), Sr­(CO)8 (2), and Ba­(CO)8 (3). Theoretical calculations reveal that M–CO interactions in these complexes can be properly described as highly polar bonds, showing some features traditionally associated with transition-metal bonding, although with noticeable differences, as well. In this sense, δ­(M–C) and δ­(M···O) delocalization indexes for bonding and nonbonding interactions, electron localization funcion (ELF) analyses, source function (SF) calculations, and the interacting quantum atoms (IQA) approach, among other methodologies, produce results consistent with interactions dominated by electrostatics between the CO ligands and alkaline-earth metals, with an increasing degree of covalency on going from 1 to 3 and without any significant π-back-donation.

The compound [Ru3(mu-H)(mu3-eta2-ampy)(CO)9] (1; Hampy =2-amino-6-methylpyridine) reacts with diynes RC4R in THF at reflux temperature to give the ynenyl derivatives [Ru3(mu3-eta2-ampy)(mu-eta3-RC...CC-CHR)(mu-CO)2-(CO)6] (2: R=CH2OPh; 3: R=Ph). These products contain a 1,4-disubstituted butynen-3-yl ligand attached to two ruthenium atoms. The compound [Ru3(mu-eta2-ampy)[mu3-eta6-PhCC5(C...CPh)-HPh2](CO)7] (4), which contains an eta5-cyclopentadienyl ring and a bridging carbene fragment, has also been obtained from the reaction of 1 with diphenylbutadiyne. This compound arises from a remarkable [3+2] cycloaddition reaction of a preformed 1,4-diphenylbutynen-4-yl ligand with a triple bond of a second diphenylbutadiyne molecule. The reactivity of the ynenyl derivatives 2 and 3 with diynes and alkynes has been studied. In all cases, compounds of the general formula [Ru3(mu-eta2-ampy)[mu3-eta5-C(=CHR)C=CRCR1=CR2](CO)7] (5-17) have been obtained. They all contain a ruthenacyclopentadienyl fragment formed by coupling of the coordinated ynenyl ligand of 2 (R = CH2OPh) or 3 (R = Ph) with a triple bond of the new reagent (the CR1=CR2 fragment results from the incoming diyne or alkyne reagent). While most of the products derived from 2 have the alkenyl C=CHR fragment with a Z configuration (R cis to Ru), all the compounds obtained from 3 have this fragment with an E configuration. Except 2 and 3, all the cluster complexes described in this article have a five-electron donor ampy ligand attached to only two metal atoms, a coordination mode unprecedented in cluster chemistry.

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The existence and characterization of a bond between the Zn atoms in the recently synthesized complex [Zn(2)(eta(5)-C(5)Me(5))(2)], as well as between Zn and ligand C atoms is firmly based on neutron diffraction and low-temperature X-ray synchrotron diffraction experiments. The multipolar analysis of the experimental electron density and its topological analysis by means of the 'Atoms in Molecules' (AIM) approach reveals details of the Zn-Zn bond, such as its open-shell intermediate character (the results are consistent with a typical metal-metal single bond), as well as many other topological properties of the compound. Experimental results are also compared with theoretical ab initio calculations of the DFT (density functional theory) and MP2 (Møller-Plesset perturbation theory) electron densities, giving a coherent view of the bonding in the complex. For instance, charges calculated from the AIM approach applied to the atomic basin of each Zn atom are, on average, +0.72 e from both the experimental and the theoretical electron density, showing a moderate charge transfer from the metal, confirmed by the calculated topological indexes.

Theoretical DFT calculations support the transient formation of carbene 2, a singular case of N-metalated NHC, derived from deprotonation of 1-phenylimidazole (L) in the cationic metal complex fac-[Mn(L)(CO)3(bipy)]+. The mechanism transforming 2 into the more stable imidazolyl tautomer 3 proceeds through the formation of the acyl intermediate 5, resulting from carbene−carbonyl coupling.

The complexes [M(κ(3)-N,H,H-mapyBH3)(cod)] (M = Rh, Ir; HmapyBH3 = 2-(methylamino)pyridine-borane; cod = 1,5-cyclooctadiene), which contain a novel anionic tripod ligand coordinated to the metal atom through the amido N atom and through two H atoms of the BH3 group, were prepared by treating the corresponding [M2(μ-Cl)2(cod)2] (M = Rh, Ir) precursor with K[mapyBH3]. X-ray diffraction studies and a theoretical Quantum Theory of Atoms in Molecules analysis of their electron density confirmed that the metal atoms of both complexes are in a very distorted trigonal bipyramidal coordination environment, in which two equatorial sites are asymmetrically spanned by the H-B-H fragment. While both 3c-2e BH-M interactions are more κ(1)-H (terminal σ coordination of the B-H bond) than κ(2)-H,B (agostic-type coordination of the B-H bond), one BH-M interaction is more agostic than the other, and this difference is more marked in the iridium complex than in the rhodium one. This asymmetry is not evident in solution, where the cod ligand and the BH3 group of these molecules participate in two concurrent dynamic processes of low activation energies (variable-temperature NMR and density functional theory studies), namely, a rotation of the cod ligand that interchanges its two alkene fragments (through a square pyramidal transition state) and a rotation of the BH3 group about the B-N bond that equilibrates the three B-H bonds (through a square planar transition state). While the cod rotation has similar activation energy in 2 and 3, the barrier to the BH3 group rotation is higher in the iridium complex than in the rhodium one.

Both triphenylphosphine ligands of the cluster complex [Rus(p-H) (p~~a-ampy)(PPh3)2(C0)7] (ampy = 2-amino-6-methylpyridinate; la) undergo a carbon-phosphorus bond scission on reaction with hydrogen (toluene, 110 "C, 1 atm) to give the 7'-phenyl-bridged derivative [Rua(p-Ph)-(p3-ampy) (p-PPh2)2(CO)61 (2a). The compound 2a.CHzC12 has been characterized by X-ray crystallography. An extended Hiickel molecular orbital calculation describing the interaction of the bridging phenyl group with the trimetallic fragment is also reported. The new complexes [Ru3(p-H)(rc3-ampy)(ER3)2(C0)71 (ER3 = PMe2Ph (Ib), PCy3 (IC), P(p-tolyh (Id), AsPh3 (le)) have been prepared in order to compare their behavior toward hydrogen with that of complex la; only Id and le gave products analogous to 2a, whereas l b and IC (unlike the others, IC is not isostructural with la) gave mixtures of unidentified products. Another ql-phenyl-bridged compound, namely, [Ru&-Ph) (p3-mbim)(p-PPh2)2(CO)el (mbim = 2-mercaptobenzimidazolate) has been obtained, without the use of hydrogen, from the reaction of [Ru&H) (pa-mbim)(CO)~] with PPh3 in refluxing THF.