A combined experimental/theoretical study gives strong evidence that carbodiphosphoranes are divalent carbon(0) compounds. The calculations show that carbodiphosphoranes have two lone pairs of electrons (see picture), which give rise to unusual properties as confirmed by experiment. The synthesis of a triply charged molecules in which two protonated carbodiphosphoranes serve as donor ligands to an Ag+ center supports the bonding model.

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We report about the first X-ray structure analyses of the CS2 and CO2 adducts with carbodiphosphorane C(PPh3)2 and the synthesis and X-ray structure analysis of group 6 carbonyl complexes with compound S2CC(PPh3)2 as a ligand [(CO)4MS2CC(PPh3)2] (M = Cr, Mo, W). The nature of the carbon-carbon bonding in X2CC(PPh3)2 and in the model compounds X2CC(PH3)2 and the metal-ligand bonding in [(CO)4MoS2CC(PH3)2] have been analyzed with charge and energy decomposition methods using DFT calculations. Carbodiphosphoranes C(PR3)2 are double electron pair donors having sigma- and pi-carbon lone-pair orbitals as the two highest occupied MOs.

Eine theoretisch/experimentelle Untersuchung liefert Hinweise, dass Carbodiphosphorane als Verbindungen mit zweibindigem Kohlenstoff(0)‐Atom einzuordnen sind. Carbodiphosphorane haben zwei freie Elektronenpaare (siehe Schema), die den Molekülen ungewöhnliche Eigenschaften verleihen. Die Synthese eines dreifach geladenen Moleküls, bei dem zwei protonierte Carbodiphosphorane als Elektronendonoren für Ag+ wirken, stützt dieses Bindungsmodell.

The carbodiphosphorane Ph3PCPPh3 (1) readily reacts with Ni(CO)4 in toluene to give the substitution product (CO)3NiC(PPh3)2 (2). If the reaction is carried out in THF solution, additionally red crystals of the dicarbonyl complex (CO)2NiC(PPh3)2 (3) are formed. 2 smoothly converts into 3 when dissolved in THF. The compounds have been characterized by single-crystal X-ray diffraction. Quantum chemical calculations at the DFT level of theory (B3LYP) are given for the geometries of model compounds 1a−3a with PH3 ligands instead of PPh3, which are in good agreement with the experimental results for 1−3. The metal−ligand bond energies are also predicted at B3LYP. The calculated Ni−C(PH3)2 bond energy of 3a (D 0 = 33.7 kcal/mol) is nearly the sum of the Ni−C(PH3)2 (D 0 = 20.9 kcal/mol) and first Ni−CO bond energy of 2a (D 0 = 16.3 kcal/mol). The analysis of the metal−ligand bonding using the CDA method shows that there is mainly ligand → metal donation and much less metal → ligand back-donation between Ni and C(PH3)2 in 2a. Donation and back-donation become stronger and back-donation becomes more important in 3a than in 2a.