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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