Carbon monoxide (1 atm) quantitatively converts the Pt(II) dinuclear derivative Pt-2[mu-P(t-Bu)(2)](2)(H)(2)[P(t-Bu)(2)H](2) (1) into the new (Pt2PtII)-Pt-I triangulo cluster Pt-3[mu-P(t-Bu)(2)](3)(H)(CO)(2) (2). The 44e(-) species 2 was characterized by IR and multinuclear NMR spectroscopy and by a single-crystal X-ray diffraction study. Complex 2 is formed through the slow CO-induced reductive elimination of P(t-Bu)(2)H from 1, affording the intermediate mononuclear Pt(0) carbonyl derivative Pt[P(t-Bu)(2)H](2)(CO)(2) (4), which equilibrates with the carbonyl-bridged triangulo derivative Pt-3[P(t-Bu)(2)H](3)(mu-CO)(3) (5); these are the main products under high pressures of CO. Two alternative mechanisms were examined for the subsequent formation of complex 2, the first being the rapid condensation of complex 4 with unreacted 1 while the second assumes 5 as the direct precursor of 2. Although the first mechanism cannot be conclusively rejected, the second seems the most appropriate, since under high pressures of CO/H-2 complex 2 was shown to equilibrate with 5. In the presence of an excess of other phosphines, the carbonylation of 1 yields quantitatively the mononuclear monocarbonyl derivatives Pt(PR(3))(3)(CO) (R(3) = Ph(3), Et(3), t-Bu(2)H).