W2(μ-CSiMe3)2(CH2SiMe3)4(I) reacts with t-BuOH in alkane solvents to produce a mixture of two isomers of W2(μ-CSiMe)2(O-t-Bu)2(CH2SiMe3)2from which the minor isomer can be isolated by fractional crystallization. the electronic structures of several related dimetallacyclobutadiene derivatives have been investigated by UV photoelectron spectroscopy, UV/visible spectrophotometry, and MO calculations. the M-M bonding configurations for the M2(μ-CSiMe3)2(CH2SiMe3)4molecules have been determined as σ°δ*°, σ2δ*0, and σ2δ*2for M = Ta, W, and Re, respectively, by EHMO and Fenske-Hall MO calculations. This result is better reconciled with the previously observed M-M distances—Nb2> Re2> W2—than with the original proposal of a σ 2 π 2 configuration for the Re derivative. On replacement of the terminal alkyl ligands of the W complex by O-i-Pr ligands, the 2auLUMO (W-W δ*) and 2b2g w- μ -c π bond are strongly destabilized by antibonding interactions with pπ orbitals on oxygen. Destabilizaμon of the 2b2gorbital results in the appearance of a clearly defined ionization representing this orbital at 7.2 eV in the PE spectrum of W2(μ-CSiMe3)2(0-i-Pr)4. Mulliken population analyses of the 6ag orbitals of I and W2(μ-CSiMe3)2(0-i-Pr)4 correctly predict that I prefers a shorter W-W distance of 2.54 A, while the alkoxide-substituted compound prefers the longer W-W distance of 2.62 A.