Exhibit 5.1

[Letterhead of King & Spalding LLP]

September 3, 2025

HCM II Acquisition Corp.

100 First Stamford Place, Suite 330

Stamford, CT 06902

Ladies and Gentlemen:

We have acted as counsel to HCM II Acquisition Corp., a Cayman Islands exempted company (the “Company”), in connection with the Registration Statement (as defined below) relating to, among other things, that certain Business Combination Agreement, dated March 26, 2025 (as it may be amended, supplemented or otherwise modified from time to time, the “Business Combination Agreement”), by and among the Company, Terrestrial Energy Inc., a Delaware corporation (“Terrestrial Energy”) and HCM II Merger Sub Inc., a Delaware corporation (“Merger Sub”).

Pursuant to the Business Combination Agreement, the Company will change its jurisdiction of incorporation to Delaware (the “Domestication”, and the effective time of the Domestication, the “Domestication Effective Time”) by effecting a deregistration under Section 206 of the Companies Act (As Revised) of the Cayman Islands and domesticate as a Delaware corporation in accordance with Section 388 of the General Corporation Law of the State of Delaware, as amended (the “DGCL”) by filing a certificate of corporate domestication together with a certificate of incorporation, in each case, in respect of the Company with the Secretary of State of the State of Delaware (the “Delaware Secretary of State”). The Domestication is subject to the approval of the Company’s shareholders. In connection with the transactions contemplated by the Business Combination Agreement (collectively, the “Transactions”), the Company will be renamed “Terrestrial Energy Inc.” and is referred to herein, following the effectiveness of the Domestication, as “New Terrestrial Energy.”

At least one day following the Domestication, Merger Sub will merge with and into Terrestrial Energy (the “Merger”, and the effective time of the Merger, the “Effective Time”), with Terrestrial Energy being the surviving corporation and continuing as a wholly-owned subsidiary of New Terrestrial Energy. In connection with the Domestication: (i) each of the then issued and outstanding Class A ordinary shares of the Company, par value $0.0001 per share (“HCM II Class A Ordinary Shares”) will convert, on a one-for-one basis, into one share of New Terrestrial Energy common stock, par value $0.0001 per share (each, a share of “New Terrestrial Common Shares”); (ii) each of the then issued and outstanding warrants representing the right to purchase one HCM II Class A Ordinary Share (the “HCM II Warrants”), issued pursuant to that certain Warrant Agreement by and between the Company and Continental Stock Transfer & Trust Company, dated August 19, 2024 (the “Warrant Agreement”), will convert automatically into a warrant to acquire one share of New Terrestrial Common Shares (each a “New Terrestrial Warrant”); and (iii) each of the then issued and outstanding units of the Company (each an “HCM II Unit”) will be canceled and each holder of such HCM II Unit will be entitled to one share of New Terrestrial Common Shares and one-half of one New Terrestrial Warrant. Following the Domestication and in connection with the consummation of the Transactions, New Terrestrial Energy will also issue shares of New Terrestrial Common Shares to holders of securities of Terrestrial Energy (the “Terrestrial Energy Securityholders”) in accordance with the terms and subject to the conditions of the Business Combination Agreement.

In rendering the opinions stated herein, we have examined and relied upon the following:

In rendering our opinion, we have examined and relied upon the accuracy of original, certified, conformed or photographic copies of such records, agreements, certificates and other documents as we have deemed necessary or appropriate to enable us to render the opinions hereinafter expressed. In making such examination and in rendering the opinions set forth below, we have assumed the genuineness of signatures on original documents and the conformity to such original documents of all copies submitted to us as certified, conformed or photographic copies and, as to certificates of public officials, we have assumed the same to have been properly given and to be accurate. As to matters of fact material to this opinion letter, we have relied, without independent verification, upon certificates and representations of representatives of the Company and public officials.

The opinions stated herein presume that:

Based upon the foregoing and subject to the qualifications and assumptions stated herein, we are of the opinion that:

The opinions stated herein are subject to the effects of (a) bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and similar laws affecting the rights and remedies of creditors generally, (b) general equitable principles (whether considered in a proceeding in equity or at law), and (c) an implied covenant of good faith and fair dealing. We express no opinion as to whether, or the extent to which, the laws of any particular jurisdiction apply to the subject matter hereof.

The opinions expressed herein are limited in all respects to the federal laws of the United States of America, laws of the State of New York and the Delaware General Corporation Law, and no opinion is expressed with respect to the laws of any other jurisdiction or any effect which such laws may have on the opinions expressed herein. This opinion is limited to the matters stated herein, and no opinion is implied or may be inferred beyond the matters expressly stated herein.

This opinion is given as of the date hereof, and we assume no obligation to advise you after the date hereof of facts or circumstances that come to our attention or changes in law that occur, which could affect the opinions contained herein. This opinion is being rendered for the benefit of the Company in connection with the matters addressed herein.

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Registration Statement. In giving such consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended.