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 HCCR; 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)
x
O)L2(η-C7H7)]+ (x = 1, L2 = dppm (1), dppe (2); x = 2, L2 = dppm (3), L2 = dppe (4)), the vinylidenes [Mo{CC(H)(CH2)2CH2OH}(dppe)(η-C7H7)][PF6] (5), [Mo{CC(H)(CH2)xPh}(dppe)(η-C7H7)]+ (x = 2 (6), 1 (7)), and [Mo{CC(H)Ph}{P(OMe)3}2(η-C7H7)]+ (8), and the first examples of monometallic molybdenum allenylidene complexes, [Mo(CCCRPh)(dppe)(η-C7H7)]+ (R = Ph (9), Me (10)) are reported. X-ray structural studies on complexes 8 and 9 have determined Mo−Cα 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(CCH2)(dppe)(η-C7H7)]+ (11), [Ru(CCH2)(dppe)Cp]+ (12), and [Mo(CCCMePh)(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
z
2
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
z
2
-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.
