A series of metallacumulenylidene complexes of the cycloheptatrienyl molybdenum auxiliary [Mo{(C)
nCR
2}L
2(η-C
7H
7)]
+ (
n = 0−2; L
2 = P-donor ligand) have been synthesized by reaction of [MoXL
2(η-C
7H
7)] (X = Br, L
2 = Ph
2PCH
2CH
2PPh
2 (dppe); X = I, L
2 = 2P(OMe)
3) or [Mo(OCMe
2)L
2(η-C
7H
7)]
+ (L
2 = dppe, Ph
2PCH
2PPh
2 (dppm)) with the terminal alkynes HC
CR; use of [MoBr(dppe)(η-C
7H
7)] as a precursor is facilitated by an improved, one-step synthesis from [MoBr(CO)
2(η-C
7H
7)]. Syntheses of the 2-oxacyclocarbene complexes [Mo(CCH
2CH
2(CH
2)
xO)L
2(η-C
7H
7)]
+ (
x = 1, L
2 = dppm (
1),
dppe (
2);
x = 2, L
2 = dppm (
3), L
2 = dppe (
4)), the vinylidenes [Mo{C
C(H)(CH
2)
2CH
2OH}(dppe)(η-C
7H
7)][PF
6] (
5), [Mo{C
C(H)(CH
2)
xPh}(dppe)(η-C
7H
7)]
+ (
x = 2 (
6),
1 (
7)), and [Mo{C
C(H)Ph}{P(OMe)
3}
2(η-C
7H
7)]
+ (
8), and the first examples of monometallic molybdenum allenylidene complexes, [Mo(C
C
CRPh)(dppe)(η-C
7H
7)]
+ (
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 MoL
2(η-C
7H
7) 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
CH
2)(dppe)(η-C
7H
7)]
+ (
11), [Ru(C
CH
2)(dppe)Cp]
+ (
12), and [Mo(C
C
CMePh)(dppe)(η-C
7H
7)]
+ (
10) concurs with experimentally determined cumulenylidene ligand orientations. A separate analysis of the fragments [Mo(dppe)(η-C
7H
7)]
+ and [Ru(dppe)Cp]
+ reveals that the HOMO of the [Mo(dppe)(η-C
7H
7)]
+ 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-C
7H
7)]
+ unit. In cumulenylidene complexes of the Mo(dppe)(η-C
7H
7) 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.