This article reports the synthesis, characterization, and chemistry of the first molybdenum and tungsten complexes bearing small carborane ligands, including novel M-M-linked dicluster species. The reaction of the nido-2, 3-Et2C2B4H42- dianion with (RCN)(3)M(CO)(3) reagents (M = Mo, W; R = Me, Et) gave the (Et2C2B4H4)M(CO)(3)(2-) anions, which were isolated as lithium (12-crown-4) salts 1b (M = Mo) and 2b (M = W). Treatment of 1b and 2b with Ph4PX (X = Cl, Br, I) followed by triflic acid afforded the dimeric products [(Et2C2B4H4)Mo(CO)(2)](2)-(mu-X)(2) (X = Cl (3a), Br (3b), I (3c)) and the corresponding tungsten dimers 4a-c, all of which are red or orange air-stable crystalline solids. X-ray crystallography on 3b revealed a geometry without precedent in small metallacarborane dimers, in which two MC2B4 pentagonal pyramidal clusters are linked via an intercluster metal-metal bond. From NMR and other spectroscopic evidence, the remaining dimeric products 3a, 3c, and 4a-c are proposed to have structures similar to 3b. Decapitation of 3b with HCl/EtOAc in THF gave red solid [(Et2C2B3H5)Mo(CO)(2)](2)(mu-Br)(2) (5), which is proposed to have two open planar C2B3 end rings and, hence, is a potential building block for linear polymers via deprotonation and metal-ion-stacking reactions. Compound 3b was also generated in a different reaction, involving the nido-Et2C2B4H5- anion and [Mo(CO)(4)Br](2)[mu-Br](2) which also produced the bis(carboranyl)molybdenum complex (Et2C2B4H4)(2)Mo(CO)(2) (6), a nearly colorless solid for which a bent sandwich structure is postulated. The reaction of the nido-cobaltacarborane anion Cp*Co(Et2C2B3H4)(-) with [Mo(CO)(4)Cl](2)[mu-Cl](2) in toluene gave two isolable products, red Cp*Co(Et2C2B3H3)Mo(CO)(4) (7) and dark red [Cp*Co(Et2C2B3H3)](2)Mo(CO)(2) (8), which have, respectively, triple-decker and bent-tetradecker sandwich structures based on X-ray crystallographic structure determinations. The reaction of Cp*Co(Et2C2B3H4)(-) with [Mo(CO)(4)Br](2)[mu-Br](2) in toluene gave 8 and a known fused-cluster complex Cp*Co-2(2)(Et4C4B6H6) (9). Treatment of the Cp*Co(Et2C2B3H4)(-) anion with [W(CO)(4)Br](2)[mu-Br](2) afforded red crystalline Cp*Co(Et2C2B3H3)W(CO)(4) (10), which on reaction with PhLi followed by Me3OBF4 gave 11, a B(5)-benzyl derivative of 10. X-ray diffraction analyses established triple-decker sandwich structures for both compounds. The new products were characterized by multinuclear NMR, IR, UV-vis, and mass spectroscopy, and elemental analysis, supplemented by electrochemical data on 3b, 8, and 11.