Metallacumulenylidene Complexes in the Cycloheptatrienyl Molybdenum Series [Mo{(C)_nCR₂}L₂(η-C₇H₇)]⁺ (n = 0−2; L₂ = P-Donor Ligand). X-ray Crystal Structures of [Mo{CC(H)Ph}{P(OMe)₃}₂(η-C₇H₇)][BF₄] and [Mo(CCCPh₂)(dppe)(η-C₇H₇)][PF₆] and DFT Investigation of Cumulenylidene Ligand Orientations

A series of metallacumulenylidene complexes of the cycloheptatrienyl molybdenum auxiliary [Mo{(C)_nCR₂}L₂(η-C₇H₇)]⁺ (n = 0−2; L₂ = P-donor ligand) have been synthesized by reaction of [MoXL₂(η-C₇H₇)] (X = Br, L₂ = Ph₂PCH₂CH₂PPh₂ (dppe); X = I, L₂ = 2P(OMe)₃) or [Mo(OCMe₂)L₂(η-C₇H₇)]⁺ (L₂ = dppe, Ph₂PCH₂PPh₂ (dppm)) with the terminal alkynes HCCR; use of [MoBr(dppe)(η-C₇H₇)] as a precursor is facilitated by an improved, one-step synthesis from [MoBr(CO)₂(η-C₇H₇)]. Syntheses of the 2-oxacyclocarbene complexes [Mo(CCH₂CH₂(CH₂)_xO)L₂(η-C₇H₇)]⁺ (x = 1, L₂ = dppm (1), dppe (2); x = 2, L₂ = dppm (3), L₂ = dppe (4)), the vinylidenes [Mo{CC(H)(CH₂)₂CH₂OH}(dppe)(η-C₇H₇)][PF₆] (5), [Mo{CC(H)(CH₂)_xPh}(dppe)(η-C₇H₇)]⁺ (x = 2 (6), 1 (7)), and [Mo{CC(H)Ph}{P(OMe)₃}₂(η-C₇H₇)]⁺ (8), and the first examples of monometallic molybdenum allenylidene complexes, [Mo(CCCRPh)(dppe)(η-C₇H₇)]⁺ (R = Ph (9), Me (10)) are reported. X-ray structural studies on complexes 8 and 9 have determined Mo−C_α distances as follows: 8, 1.929(3) Å; 9, 1.994(3) Å. In 8, the vinylidene ligand substituents lie in the pseudo mirror plane of the MoL₂(η-C₇H₇) auxiliary (vertical orientation) with the phenyl group located syn to the cycloheptatrienyl ring, whereas the allenylidene ligand substituents of 9 are perpendicular to the pseudo mirror plane (horizontal orientation). This series of atypical orientations of cumulenylidene ligand substituents, imposed by the cycloheptatrienyl molybdenum auxiliary, has been investigated further by variable-temperature NMR spectroscopy and quantum-chemical theoretical calculations. A DFT analysis of the complexes [Mo(CCH₂)(dppe)(η-C₇H₇)]⁺ (11), [Ru(CCH₂)(dppe)Cp]⁺ (12), and [Mo(CCCMePh)(dppe)(η-C₇H₇)]⁺ (10) concurs with experimentally determined cumulenylidene ligand orientations. A separate analysis of the fragments [Mo(dppe)(η-C₇H₇)]⁺ and [Ru(dppe)Cp]⁺ reveals that the HOMO of the [Mo(dppe)(η-C₇H₇)]⁺ fragment includes a significant contribution from the metal d_z² orbital, whereas the HOMO of the [Ru(dppe)Cp]⁺ fragment is based on a metal d_xy orbital, orthogonal to the HOMO of the [Mo(dppe)(η-C₇H₇)]⁺ unit. In cumulenylidene complexes of the Mo(dppe)(η-C₇H₇) auxiliary, interactions between the d_z²-based HOMO of the metal fragment and the vacant LUMO of the cumulenylidene ligand dominate the control of ligand orientation and thus account for the observed structures.