Metallacumulenylidene [Mo{(:C)n:CR2}L2(h-C7H7)]+ (R = H, Me, Ph, CH2Ph, (CH2)2Ph, (CH2)3OH; n = 1, 2; L2 = P-donor ligand) and 2-oxacyclocarbene complexes of the cycloheptatrienyl Mo auxiliary were synthesized by reaction of [MoXL2(h-C7H7)] (X = Br, L2 = Ph2PCH2CH2PPh2 (dppe); X = I, L2 = 2P(OMe)3) or [Mo(OCMe2)L2(h-C7H7)]+ (L2 = dppe, Ph2PCH2PPh2 (dppm)) with the terminal alkynes HC==CR (R = (CH2)2OH, (CH2)3OH, CH2Ph, (CH2)2Ph, Ph, CPh2OH, CMePhOH). The use of [MoBr(dppe)(h-C7H7)] as a precursor is facilitated by an improved, 1-step synthesis from [MoBr(CO)2(h-C7H7)]. Syntheses of the 2-oxacyclocarbene complexes [Mo(:CCH2CH2(CH2)xO)L2(h-C7H7)]+ (x = 1, L2 = dppm (1), dppe (2), shown as I; x = 2, L2 = dppm (3), L2 = dppe (4), shown as II), the vinylidenes [Mo{:C:C(H)(CH2)2CH2OH}(dppe)(h-C7H7)][PF6] (5), [Mo{:C:C(H)(CH2)xPh}(dppe)(h-C7H7)]+ (x = 2 (6), 1 (7)), and [Mo{:C:C(H)Ph}{P(OMe)3}2(h-C7H7)]+ (8), and the 1st examples of monometallic Mo allenylidene complexes, [Mo(:C:C:CRPh)(dppe)(h-C7H7)]+ (R = Ph (9), Me (10)) are reported. X-ray structural studies on complexes 8 and 9 detd. Mo-Calpha distances as follows: 8: 1.929(3) .ANG.; 9: 1.994(3) .ANG. In 8, the vinylidene ligand substituents lie in the pseudo mirror plane of the MoL2(h-C7H7) auxiliary (vertical orientation) with the Ph 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 Mo auxiliary, was studied further by variable-temp. NMR spectroscopy and quantum-chem. theor. calcns. A DFT anal. of [Mo(:C:CH2)(dppe)(h-C7H7)]+ (11), [Ru(:C:CH2)(dppe)Cp]+ (12), and [Mo(:C:C:CMePh)(dppe)(h-C7H7)]+ (10) concurs with exptl. detd. cumulenylidene ligand orientations. A sep. anal. of the fragments [Mo(dppe)(h-C7H7)]+ and [Ru(dppe)Cp]+ reveals that the HOMO of the [Mo(dppe)(h-C7H7)]+ fragment includes a significant contribution from the metal dz2 orbital, whereas the HOMO of the [Ru(dppe)Cp]+ fragment is based on a metal dxy orbital, orthogonal to the HOMO of the [Mo(dppe)(h-C7H7)]+ unit. In cumulenylidene complexes of the Mo(dppe)(h-C7H7) auxiliary, interactions between the dz2-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 obsd. structures. [on SciFinder (R)] ; CAN 148:379715 29-11 Organometallic and Organometalloidal Compounds Journal 0276-7333 written in English. 1013394-53-0P; 1013394-54-1P Role: PRP (Properties), SPN (Synthetic preparation), PREP (Preparation) (crystal structure; prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations); 1013394-48-3P; 1013394-50-7P Role: PRP (Properties), SPN (Synthetic preparation), PREP (Preparation) (mol. structure; prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations); 1013394-62-1P Role: BYP (Byproduct), PREP (Preparation) (prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations); 2143-69-3 (Ethenylidene); 70571-89-0 (1,2,3-Butatrien-1-ylidene); 121195-06-0; 129066-04-2 (1,2,3,4-Pentatetraenylidene); 171415-71-7; 404835-25-2; 849148-92-1; 1013394-51-8; 1013394-58-5; 1013394-59-6 Role: PRP (Properties) (prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations); 1013394-52-9P Role: PRP (Properties), SPN (Synthetic preparation), PREP (Preparation) (prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations); 127-66-2 (2-Phenyl-3-butyn-2-ol); 536-74-3 (Phenylethyne); 927-74-2 (3-Butyn-1-ol); 3923-52-2 (1,1-Diphenyl-2-propyn-1-ol); 5390-04-5 (4-Pentyn-1-ol); 10147-11-2 (3-Phenyl-1-propyne); 16520-62-0 (4-Phenyl-1-butyne); 52326-38-2 ((h7-Cycloheptatrienyl)(h6-toluene)molybdenum hexafluorophosphate); 53556-07-3 (Bromodicarbonylcycloheptatrienylmolybdenum); 123384-16-7 Role: RCT (Reactant), RACT (Reactant or reagent) (prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations); 183135-34-4P; 1013394-56-3P; 1013394-57-4P; 1013394-60-9P Role: RCT (Reactant), SPN (Synthetic preparation), PREP (Preparation), RACT (Reactant or reagent) (prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations); 183135-22-0P; 183135-24-2P; 183135-28-6P; 183135-30-0P; 1013394-44-9P; 1013394-46-1P Role: SPN (Synthetic preparation), PREP (Preparation) (prepn. and structure of cycloheptatrienylmolybdenum oxacyclocarbenes, vinylidenes and allenylidenes via cycloheptatrienylmolybdenum complexes and terminal alkynes and DFT study of cumulenylidene orientations)