η¹-Ligation versus Pnictogen−Carbon Double Bond Cleavage:  The Contrasting Behavior of Phospha- and Arsaalkenes [Tp*(CO)₂M⋮C−EC(NMe₂)₂] [E = P, As; M = Mo, W; Tp* = HB(3,5-Me₂pz)₃] toward [Au(CO)Cl]

Reaction of [Tp*(CO) 2 M⋮C−P C(NMe 2 ) 2 ] [ 1a, M = Mo; 1b, W; Tp* = HB(3,5-Me 2 pz) 3 ] with 2 molar equiv of [Au(CO)Cl] afforded the trinuclear complexes [Tp*(CO) 2 M⋮C−P(AuCl) 2 C(NMe 2 ) 2 ] 2a (M = Mo) and 2b (M = W). In marked contrast to this the arsenic analogues [Tp*(CO) 2 M⋮C−As C(NMe 2 ) 2 ] ( 3a, M = Mo; 3b, M = W), when treated with the gold complex, give rise to As C cleavage with formation of the gold carbene complex [ClAuC(NMe 2 ) 2 ] ( 5 ) and the functionalized cyclotriarsanes [{Tp*(CO) 2 M⋮C−As} 3 ] 4a (M = Mo) and 4b (M = W). Compounds 2a, b and 4a, b have been characterized by IR and 1 H and 13 C NMR spectroscopy. In addition, the molecular structures of 2a, 2b, and 5 have been determined by single-crystal X-ray structural analyses.