Addition of dialkylzinc reagents to [PPh(4)](2)[OsO2Cl4] gives OsO(2)R(2) complexes (R = neopentyl, neophyl, or (trimethylsilyl)methyl). The reaction between OsO2(CH2-t-Bu)(2) and 2 equiv of Ta(CH-t-Bu)(CH2-t-Bu)(3) gave spn, anti-Os(CH-t-Bu)(2)(CH2-t-Bu)(2), while [OsO2(CH(2)SiMe(3))(2)](n) reacted to give a 1:1 mixture of syn,anti-Os(CH-t-Bu)(2)(CH(2)SiMe(3))(2) and anti,anti-Os(CH-t-Bu)(2)(CH(2)SiMe(3))(2). Isolated anti,anti-Os(CH-t-Bu)(2)(CH(2)SiMe(3))(2) was shown in an X-ray study to be a distorted tetrahedron in which the two neopentylidene ligands lie in the same plane. Extended Huckel calculations performed on Os(CH2)(2)(CH3)(2) suggest that the HOMO is an orbital of primarily d(Z)(2) character that lies between and in the plane of the OsC2 fragment, the LUMO is the pi* system of the bis(methylidene), and the HOMO-1 and HOMO-2 are bonding combinations of the n system. Os(CH-t-Bu)(2)(CH2-t-Bu)(2) reacts with neat trimethylphosphine to give Os(PMe(3))(3)(eta(2)-t-BuC=C-t-Bu) which was shown in an X-ray study to be a pseudotetrahedral species in which C=C = 1.30 (1) Angstrom. syn,anti-Os(CH-t-Bu)(2)(CH2-t-Bu)(2) reacts with pyridinium triflate in the presence of pyridine to yield Os(C-t-Bu)(CH2-t-Bu)(2)(py)(2)(OTf), while OsO2(CH2-t-Bu)(2) reacts with 1 equiv of Ta(CH-t-Bu)(py)(2)X(3) (X = Cl, Br) to form Os(C-t-Bu)(CH2-t-Bu)(py)(2)X(2). Os(C-t-Bu)(CH2-t-Bu)(2)L (L = O-t-Bu or N(SiMe(3))(2)), Os(C-t-Bu)(CH2-t-Bu)(2){CpCo[P(O)(OEt)(2)](3)} and Os(C-t-Bu)(CH2-t-Bu)(2)[HB(pyrazolyl)(3)] could all be prepared from Os(C-t-Bu)(CH2-t-Bu)(2)(py)(2)(OTf). An X-ray study of Os(C-t-Bu)(CH2-t-Bu)(2)(HBpz(3)) showed it to be a pseudooctahedral molecule in which Os=C = 1.73(2) Angstrom.