Here's the applicable section of the SEC S-4:
"Tax Consequences of the Merger to Holders of Praxair Shares
Tax Consequences to U.S. Holders
The merger is expected to be an exchange described in Section 351(a) of the Code and a “reorganization” within the meaning of Section 368(a) of the Code for U.S. federal income tax purposes. However, with respect to cross-border exchanges such as the merger, Section 367(a) of the Code and the U.S. Treasury Regulations promulgated thereunder generally require U.S. shareholders to recognize gain (but not loss) if stock of a domestic corporation is exchanged for stock of a foreign corporation and the U.S. shareholders receive more than 50% (by vote or value) of the stock of the foreign corporation. Praxair shareholders will receive more than 50% of the Linde plc shares. Consequently, if a U.S. holder receives Linde plc shares with a fair market value in excess of the adjusted tax basis of such U.S. holder’s Praxair shares exchanged in the merger, such U.S. holder will recognize capital gain, if any, to the extent of the difference. Subject to the discussion of fractional shares below, a U.S. holder will not recognize any loss on Praxair shares exchanged in the merger. Any such gain would be long-term capital gain if the holding period for Praxair shares exceeds one year at the effective date of the merger. Long-term capital gains of certain non-corporate U.S. holders, including individuals, generally are eligible for preferential rates of U.S. federal income tax. If a U.S. holder acquired different blocks of Praxair shares at different times or different prices, such U.S. holder must determine its adjusted tax basis and holding period separately with respect to each block of Praxair shares.
A U.S. holder who recognizes gain with respect to all of its Praxair shares exchanged in the merger will have an aggregate tax basis in the Linde plc shares received in the merger (including any fractional entitlement to Linde plc shares deemed received and exchanged for cash, as described below) that is equal to the fair market value of the Linde plc shares as of the effective date of the merger. In the case of a U.S. holder who does not recognize gain with respect to any of its Praxair shares exchanged in the merger, the aggregate basis of the Linde plc shares received in exchange for Praxair shares in the merger (including any fractional Linde plc shares deemed received and exchanged for cash, as described below) will be equal to the basis of Praxair shares exchanged. The holding period of Linde plc shares received by a U.S. holder will include the holding period of the Praxair shares surrendered in exchange therefor. U.S. holders who recognize gain only with respect to certain blocks of Praxair shares should consult their own tax advisors as to the determination of the bases and holding periods of the Linde plc shares received in the merger."
Bottom line: If you had a gain, you recognize and declare the gain. If you have a loss, it carries over to the new Linde Shares.
