An unstable bis(alkylidene) complex (RCH 2 ) 4 Ta 2 (dCHR) 2 (Cl) 2 (R ) SiMe 3 , 1), prepared by the addition of 2 equiv of HCl to a symmetrically bridging bis(alkylidyne) complex (RCH 2 ) 4 -Ta 2 (µ-CR) 2 (2a), decomposes through elimination of SiMe 4 to form an unstable alkylidenealkylidyne complex (RCH 2 ) 3 Ta 2 (dCHR)(tCR)(Cl) 2 (3). This conversion of an alkylidene to alkylidyne ligand was found to follow first-order kinetics with ∆H 1 q ) 14.1(0.8) kcal/mol and ∆S 1 q ) -12(3) eu. This is, to our knowledge, the second reported kinetic study of alkylidene f alkylidyne conversion, and the kinetics of this conversion is in contrast to that of (RCH 2 ) 3 TadCHR f (RCH 2 ) 4 Ta 2 (µ-CR) 2 (2a) reported earlier. Substitution of the chlorides by SiR 3 -(R ) SiMe 3 ) leads to a preferential elimination of HSiR 3 and formation of the first silyl bis(alkylidyne) complex (RCH 2 )(R 3 Si)Ta(µ-CR) 2 Ta(CH 2 R) 2 (4). Addition of excess PMe 3 to 1 produces a novel 1,1′-dimetallacyclobutadiene derivative (Me 3 P) 2 (Cl)Ta(µ-CR) 2 Ta(Cl)-(CH 2 R) 2 (5a) with two nonsymmetrically bridging alkylidyne ligands. Both bridging ligands coordinate with CdTa double bonds to the same metal atom (and with C-Ta single bonds to the other metal atom). The two bridging alkylidyne ligands in 5a, which are coordinated to one metal atom in the axial and equatorial position, respectively, are involved in an intramolecular ligand exchange. The kinetic barriers for this exchange were determined to be ∆H 2 q ) 13.6(0.4) kcal/mol and ∆S 2 q ) -5(2) eu in 5a, and ∆H 3 q ) 13.0(0.3) kcal/mol and ∆S 3 q ) -5(1) eu in 5b. Addition of 2 equiv of RCH 2 Li to replace the Cl ligands in 5a converts 5a to 2a, completing a reaction cycle 2a f 1 f 5a f 2a. (Me 3 P) 2 (RCH 2 )Ta(µ-CR) 2 Ta(Cl)-(CH 2 R) 2 (6) was identified as an intermediate in the conversion of 5a to 2a. absences, were uniquely consistent with the space group P21/ n. The structure was solved using the Siemens SHELXTL 93 (Version 5) proprietary software package. The niobium atoms were found by the Patterson method. The structure was completed by successive Fourier syntheses and refined to convergence by successive full-matrix least-squares refinements of F 2 . All non-hydrogen atoms were anisotropically refined. The hydrogen atoms were placed in calculated positions and introduced into the refinement as fixed contributors with isotropic U values of 0.08 Å 2 . Crystal, data collection, and refinement parameters are given in Table 1.